A woman asks: The normal monthly menstrual blood showed after the opening of the fast and before I performed the Maghrib prayer; so is my fasting correct?
Yes, it is correct. As long as it did not come upon you except after sunset, then it is not incumbent upon you to repeat (that day’s fasting), and that day’s fasting is correct. As for if the (menstrual) blood comes upon you before the adhaan, meaning before the setting of the sun, then (your) fasting on that day is ruined and you must repeat that day’s fasting. But as long as the (menstrual) blood came after the setting of the sun and after you have taken food for the opening of the fast, then the fasting of that day is correct and it is not incumbent upon you to (repeat it) Inshaa-Allaah.
[For further details: see: Al-Fataawaa Wad-Duroos Fil-Masjid Al-Haraam of Shaikh Abdullaah Bin Muhammad Bin Humaid (rahimahullaah) page: 476-477]