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 'alyhi wa sallam, is His slave and Messenger.
Shaykh Ibn ‘Uthaymeen said: "Her fasting on that day is valid as the basic principle is that there is no menstruation until she is sure that this is menstruation."
Allaah Knows best.