If a woman sees blood but is not sure whether it is menstruation or not, what is the ruling on her fasting on that day?

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.