If a woman sees blood on one day during the days of menstruation and on the following day she does not find any blood all day, what should she do?

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: "It appears that this purity or dryness that takes place during the days of menstruation is to be included in the days of menstruation and so it is not to be considered purity. Thus, she should abandon at that time all that is abandoned by women during menstruation. Some scholars have said that the one who sees blood one day but is pure the next should consider the day when she sees blood as menstruation and the other day as purity until the fifteenth day. After fifteen days, she should consider any blood to be vaginal bleeding outside the menstrual period. This is the well-known opinion of Imam Ahmad ibn Hanbal ."

Allaah Knows best.