Is she still menstruating if she sees a spot of blood and a brownish discharge after performing ghusl following her period, and are her prayers valid or does she have to make them up?

If a woman is certain that menstrual blood has exited, she must perform ghusl (ritual bath). However, if she is in doubt, nothing is required of her. Discolored discharge (kudrah) is considered menstruation if it occurs during the customary period; otherwise, it is not. She must cleanse herself from it (istinja') and repeat her ablution (wudu').