She is going to go to Jeddah and she does not have any firm intention to do ‘umrah; from where should she enter ihram if she does decide to do ‘umrah?

Firstly: 

If a person passes the miqaat intending to do Hajj or ‘umrah,
it is not permissible for him to pass it without entering ihram, because of
the report narrated by Ibn ‘Abbaas (may Allah be pleased with him) who said:
The Messenger of Allaah (peace and blessings of Allaah be upon him) defined
the meeqaat of the people of Madeenah as Dhu’l-Hulayfah; that of the people
of Shaam (Syria) as al-Juhfah; that of the people of Najd as Qarn
al-Manaazil; and that of the people of Yemen as Yalamlam. And he said: “And
these miqaats are for the people at those very places, and besides them for
those who come through those places, and are not residents thereof, with the
intention of performing Hajj and ‘Umrah; and whoever is living within these
boundaries can enter ihraam from the place where he lives, and the people of
Makkah can start from Makkah.”

Narrated by al-Bukhaari, 1526; Muslim, 1181 

An-Nawawi (may Allah have mercy on him) said: ash-Shaafa‘i
and our companions said: If the one who has come from far away comes to the
miqaat intending to perform Hajj or ‘Umrah or qiraan (i.e., both), it is
haraam for him to pass it without entering ihram, according to scholarly
consensus.

End quote from al-Majmoo‘, 7/214 

But if he passed the miqaat without intending to do Hajj or
‘umrah, or he was not sure and had not yet made up his mind, then he decided
to do ‘umrah after passing the miqaat, he should enter ihram from the place
where he decided to do ‘umrah, unless he is in Makkah, in which case he
should go outside of the Haram zone then enter ihram for ‘umrah from there. 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said,
commenting on the hadeeth of Ibn ‘Abbaas quoted above: 

From this we understand that if a person travels without the
intention of doing Hajj or ‘umrah and he passes the miqaat, then after that
he decides to do Hajj or ‘umrah, he should enter ihram from the place where
he decided to do Hajj or ‘umrah, and he does not have to go back to the
miqaat, because the Prophet (blessings and peace of Allah be upon him) said:
“from the place he sets out”.

End quote from Fath al-Baari

Shaykh Muhammad al-Mukhtaar ash-Shanqeeti (may Allah preserve
him) said: 

The second scenario: if he went to Jeddah and was hesitant,
thinking: I do not know if I will have enough time or not, then in such
cases it is permissible for him not to enter ihram at the miqaat of Madinah. 

For example: there is a person who has some business to do in
Jeddah and he does not know whether he will have enough time to do ‘umrah or
not. If he is not sure about the time, it is permissible for him to go to
Jeddah without ihram, and do what he needs to in Jeddah, then enter ihram
from Jeddah if he decides to go to ‘umrah from there, because the Prophet
(blessings and peace of Allah be upon him) said: “and whoever is living
within these boundaries can enter ihraam from the place he decides (to do
‘umrah).” 

This applies to the one who does not have the clear intention
of performing ‘umrah from Madinah, so he is uncertain and hesitant; he does
not have to enter ihram until he is certain that he is going to do ‘umrah.

End quote from Sharh Zaad al-Mustaqni‘. 

Shaykh ‘Abd al-‘Azeez ibn ‘Abdullah ibn Aal ash-Shaykh (may
Allah preserve him) was asked: 

My colleagues and I were sent to Jeddah to do work for Hajj
for two months. Some of us put our ihram in our cars or suitcases, then when
we found some free time we intended to do ‘Umrah and entered ihram from
Jeddah. Is this permissible, or do we have to go to the miqaat of as-Sayl
and enter ihram from there? 

He replied: If you had decided to do ‘umrah before
travelling, and from the time you started your journey to Jeddah your
intention was to do ‘umrah, then what you have to do is enter ihram from the
miqaat, because the intention to do ‘umrah had already been formed. But if
the decision to do ‘umrah was not made until after you had settled in
Jeddah, then you should enter ihram from Jeddah. 

But if your intention was not fixed, in the sense that you
were not sure, then this uncertainty means that you had not firmly decided
to do ‘umrah. Based on that, if you made your mind up when you were closer
(to Makkah) than the miqaat, you can enter ihram from the place where you
formed your intention. And Allah knows best. 

End quote from Majallat al-Buhooth al-Islamiyyah,
60/95 

Secondly: 

If a person passes the miqaat, intending to do Hajj or
‘umrah, then he does not enter ihram from the miqaat, he has to go back to
the miqaat that he passed and enter ihram from there. If he does not go
back, he has to offer a sacrifice for each time that he passed it. 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked about
a person who was obliged to offer a sacrifice because he entered ihram from
Jeddah after passing the miqaat, and he has made this mistake several times
– what does he have to do? Should he offer one sacrifice and will that be
sufficient, or is the answer something other than that? 

He replied: He has to offer a sacrifice for each occasion, to
be slaughtered in Makkah for the poor, if he passed the miqaat intending to
do Hajj or ‘umrah, then he entered ihram from Jeddah. One seventh of a camel
or one seventh of a cow is acceptable as a sacrifice for that. He also has
to repent to Allah, may He be glorified, from that, because it is not
permissible for a Muslim to pass the miqaat, intending to do Hajj or ‘Umrah,
without entering ihraam, because the Prophet (blessings and peace of Allah
be upon him) said: “And these miqaats are for the people at those very
places, and besides them for those who come through those places, and are
not residents thereof, with the intention of performing Hajj and ‘Umrah”;
and because Ibn ‘Abbaas (may Allah be pleased with him) said: Whoever omits
a ritual (of Hajj or ‘umrah) or forgets it, let him offer a sacrifice.

End quote from Majmoo‘ al-Fataawa, 17/12 

He has to try hard to remember how many times he has to offer
sacrifices for, until he thinks it most likely that he has discharged his
duty. 

Thirdly: 

If a woman sees the tuhr (signal of the end of menses) that
she usually sees at the end of her period, whether that is complete dryness
or the white discharge, whatever usually signals the end of her period, and
she purifies herself and does tawaaf (circumambulation of the Ka‘bah) and
sa‘i (going between as-Safa and al-Marwah), or prays and fasts, then after
that she becomes uncertain about the state of purity on which she based
those acts of worship, she should not pay any attention to that uncertainty
and she does not have to do anything, because uncertainty after completing
an act of worship does not have any impact, so long as she started it when
she was certain about her situation, on the basis of what she usually sees
at the end of her
menses.                                                                   

But if she was hasty and was not sure whether her menses had
ended before she started ‘umrah, and she did not sees what she usually sees
every time her period ends, then after that she began to doubt whether her
menses had ended, as is the case in this question, then her ‘umrah is not
complete, because the basic principle is that she was still menstruating and
she did not know whether her period has ended. So she has to avoid all the
things that are prohibited during ihram, the most important of which is
intercourse with her husband, because she is still in a state of ihram until
she goes back to Makkah, does tawaaf and saa‘i, and cuts her hair, then
exits her ihram. 

If she did any of the actions that are prohibited during
ihram before that, she is excused for them because she thought that her
‘umrah had ended. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
With regard to what she has done of the things that are prohibited (during
ihram), let us assume that her husband had intercourse with her, and
intercourse during ihram is the most serious of the prohibited actions;
however there is no blame on her because she was unaware of the ruling. If a
person does one of the actions that are prohibited during ihram out of
ignorance or because he forgot or was forced to do it, there is no blame on
him. 

End quote from Majmoo‘ al-Fataawa, 21/351 

And Allah knows best.