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: "If a woman in a state of purity feels menstrual blood beginning to flow outside the womb but it does not actually emerge until after sunset; or she feels the period cramps that precede menstruation but the menstrual blood does not actually emerge until after sunset while she is fasting, her fasting is valid and she does not have to make up for that day if it is an obligatory fast and she will receive the reward of optional fasting if that day is an optional day for fasting."
Allaah Knows best.