What should be done if a woman sees a clear red bloodstain after intercourse, having previously seen a brown discharge, then a completely white discharge after her menstruation stopped for one day, and she had already performed ghusl and prayed, but forgot her usual menstrual cycle?

If what you saw is the discharge that women see when they become pure, then this is purity during menstruation, and you have the rulings of pure women, and there is no blame on you for what you did. After the bleeding, you became a menstruating woman again, as long as it does not exceed fifteen days.