Her period is not regular

Firstly: 

Menstrual blood is dark and is well-known to women. It has particular characteristics: in terms of colour it is dark; in terms of consistency it is thick; in terms of odour it is unpleasant and has an odour different to that of ordinary blood. 

If you see that type of blood, then it is your menses, whether the duration of the bleeding is seven days or eight, or more or less, so long as it does not last for longer than fifteen days. 

Secondly: 

Brownish secretions come under the same rulings as menses if they are connected to it and come straight after it, before the woman sees the signs of tuhr (purity, i.e., the end of menses) which she knows. So they are regarded as part of the period, even if their characteristics differ from those of menstrual bleeding. 

Similarly, if the woman usually has these secretions at the beginning of her period, and they are connected to the menstrual bleeding that she knows, accompanied by pains or cramps, or they come at the usual time of her period, then they are also regarded as part of the period. 

It says in al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah (18/295): 

The majority of fuqaha’ are of the view that yellowish or brownish discharges during the time of the period are also menses, because they are what a woman usually sees at the time when her period may come. Moreover, the women used to send to ‘Aa’ishah (may Allah be pleased with her) small vessels in which there was a cloth on which there was yellowish or brownish discharge, and she would say to them: Do not be hasty, until you see the white discharge – referring to the tuhr which signals the end of the period. The yellowish and brownish discharges are something like pus. End quote.

Ibn ‘Aabideen (may Allah have mercy on him) said: 

Abu Yoosuf did not agree that the brownish discharge could be part of the menses if it appears at the beginning of the period, but he thought that that which appears at the end is menses. Some of the scholars did not regard the greenish discharge as part of the period. The correct view that it is menses in the case of women who menstruate, but not postmenopausal women.

End quote from Radd al-Muhtaar (1/289). 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

The dirty-looking discharge that appears after the end of the period, and does not appear at the time of the period itself, is not to be acted upon and no attention should be paid to it, because the brownish or yellowish discharge after tuhr which signals the end of the period is not regarded as being of any significance. It is not regarded as menses; rather it comes under the same heading as urine: the woman who experiences that has to clean herself with water (istinja’) and do wudoo’ as for prayer, and take measures to prevent it getting onto her clothing (e.g., wearing pads) when the time for each prayer begins. 

But if that discharge comes immediately after the period and is connected to it, or it comes at the beginning of the period, or at the time when the period is expected, then it is regarded as menses. In that case the woman should not pray or circumambulate the Ka‘bah (tawaaf) until she becomes pure (i.e., her period ends and she does ghusl).

End quote from Majmoo‘ Fataawa Ibn Baaz (29/116) 

Shaykh Ibn Baaz (may Allah have mercy on him) was also asked: 

Before my period begins I get a discharge that is brown in colour and lasts for five days, and after that the menstrual bleeding begins, and lasts for eight days after the first five days. She says: I pray during these five days, but I wonder whether I have to fast and pray during these days or not? Please advise me, may Allah reward you. 

He replied: 

If the five days during which the brownish discharge occurs are separate from the days when bleeding occurs, then they are not part of the menses and you have to pray and fast during these days, and do wudoo’ for each prayer, because this comes under the same rulings as urine, not the rulings on menses. So it does not prevent you from praying or fasting, but you have to do wudoo’ every time (for each prayer) until it stops, as is also the case with regard to irregular bleeding (istihaadah). 

But if these five days are connected to the period, then they come under the heading of menses and should be regarded as part of the period. In that case you should not pray or fast during those days. 

Similarly, if this brownish or yellowish discharge comes after the tuhr which signals the end of menses, then they are not regarded as menses; rather they come under the same ruling as irregular bleeding (istihaadah). You have to clean yourself with water (istinja’) every time, and do wudoo’ and pray and fast. That is not regarded as menses, and you are permissible for your husband (at that time), because Umm ‘Atiyyah (may Allah be pleased with her) said: We used not to regard the yellowish brownish discharge after the white discharge (that signals the end of the period) as being of any significance. Narrated by al-Bukhaari in his Saheeh, and by Abu Dawood. Umm ‘Atiyyah was one of the virtuous women of the Sahaabah who narrated many hadeeths from the Prophet (blessings and peace of Allah be upon him, and may Allah be pleased with her). And Allah is the source of strength.

End quote from Majmoo‘ Fataawa Ibn Baaz (10/207-208)

To sum up: whatever bleeding or secretions occur at the time of menses come under the ruling on menses, whether that occurs before the usual menstrual bleeding and is connected to it, or it comes at the end of your period, before you see the usual signs of tuhr (end of menses). 

Thus you will know that you have to refrain from fasting and praying for the entire duration of your period, whether it is seven days or ten days, so long as you have not yet seen your tuhr (sign that the period has ended). If your period ended on the 17^th, for example, then you have to purify yourself and do ghusl, and resume praying and fasting, from the time when you see your tuhr. The same applies if that happens on the 18^th. And you have to do that every month. 

Thirdly: 

If you see bleeding two weeks after the end of your period, as you mention, and it does not look like menstrual blood, then this is irregular bleeding (istihaadah) and not menses, because it does not match the description of menstrual blood and it has not come at the time of your usual period. So that should not prevent you from fasting and praying; rather you should take measures to prevent it getting onto your clothing (e.g., wear pads), and do wudoo’ for each prayer, then pray. 

You should continue to pray and fast until your period begins, and you recognise it by its signs or because it comes at the usual time of your period, as explained above. 

Fourthly: 

If you are no longer certain about any of these matters, and you no longer know the time of your period or how long it lasts, and you cannot distinguish between menstrual bleeding and other bleeding, then you should regard your period as being seven days, which is the closest thing to your previous cycle, and put it at the time when you use to get your period before it became irregular. Then (after seven days), do ghusl and pray, as the Prophet (blessings and peace of Allah be upon him) said to Hamnah bint Jahsh, when she complained to him that she was bleeding a great deal: “That is only a kick from the shaytaan, so count your menses as six or seven days, which is something known to Allah, then wash yourself and when you see that you have become pure and it is over, then pray for twenty-four or twenty-three days, and fast and pray. That will suffice you, so do what other women do with regard to their menses and purification.” Narrated by Ahmad (27474) and Abu Dawood (287), and others; classed as hasan by al-Albaani. 

To sum up: 

·Whatever bleeding or secretions occur at the time of menses come under the same ruling as menses, whether that occurs before the typical menstrual bleeding and is connected to it, or it comes at the end of the period before the signs of tuhr.

·If bleeding occurs two weeks after the end of the period, and does not match the description of menstrual bleeding, then it does not come under the same rulings as menses and does not prevent a woman from praying and fasting, until the period begins.

·If your menstrual cycle has become irregular and you are confused about the type of bleeding, and you cannot distinguish between menstrual bleeding and other types of bleeding, then regard your period as being seven days, which is the closest to your previous cycle, and regard it as beginning at the time when you used to get your period before it became irregular, then (after that) do ghusl and pray.

And Allah knows best.