Her period comes twice a month – should she stop praying and fasting each time?

The ruling depends on the reason for it; if the reason is present, the ruling applies. If there is menstrual blood which is known by its characteristics and colour, then the woman must follow the rulings on menstruation, and prayer, fasting and intercourse are forbidden at that time, even if it happens more than once in a month and even if it lasts more than the usual number of days each month. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a woman whose period came, then she became pure and did ghusl, then after she had prayed for nine days, she started to bleed again, so she did not pray for three days, then she became pure and prayed for eleven days, then her period came again as usual. Should she repeat the prayers she offered during those three days or should they be regarded as part of her period? 

He replied: 

When the period comes it is hayd (menstruation) regardless of whether a long time or a short one has passed between it and the previous period. If a woman has her period then becomes pure, then after five days, or six, or ten, her period comes again, she should stop praying because this is hayd. This applies all the time. Every time she becomes pure then her period comes again, she has to stop praying etc. But if she bleeds continually and it only stops for a short while, then she is mustahaadah (one who suffers from istihaadah or non-menstrual vaginal bleeding), and in that case she should only stop praying for the duration of her normal menses. 

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 11 Shawwaal 230. 

And Allaah knows best.