Reference: Fataawa Ramadhaan – Volume 1, Page 348, Fatwa No.298
al-Muntaqaa min Fataawa Shaykh Saalih Ibn Fowzaan – Volume 3, Page 131
Question: What is the ruling regarding a woman whose menses would begin after she made the intention to fast?
Response: If a woman’s menses began after she started fasting, then it invalidates her fast and she must refrain from fasting for the duration of her menses. [Afterwards] when her menses cease, she must fast the remainder of the month and then make up [the fasts] for the days she did not fast due to her menses.