All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Fasting has validity requirements, among which is to make the intention of fasting the night before and for women to be free of menses before the Fajr (dawn prayer time). If a menstruating woman comes to be free of menses before the Fajr but did not know this until afterwards, then she is required to make up for that day later. She cannot fast that day because she had not made the intention of fasting before Fajr time. She is not, however, required to pay an expiation, nor is she blamed for missing such a day as long as she did not miss it deliberately. However, she has to fast the rest of the day (once knowing about the expiry of her menstruation period) for she was under the obligation of fasting since the beginning of the day. She is not held sinful for her ignorance about the expiry of her menstruation period. However, if a woman is certain that her period ended and intentionally breaks her fast, then she is sinful and is required to make up for the day without paying an expiation, as stated by the majority of Muslim scholars. Some scholars, however, say that an expiation is obligatory in this case.
Allaah Knows best.