How can one know the days that must be made up from Ramadan, in case of uncertainty regarding purification from menstruation, and fast them despite the non-obligation of fasting?

A woman knows she is pure by one of two signs: dryness or the white discharge. If a woman performs ghusl (ritual bath) while doubting her purity, her ghusl is not considered valid, and her purity is not established except with certainty.

Therefore, she must make up the days she fasted while she was in doubt about her menstrual status or purity.

However, if the doubt is merely a predominant waswas (whisper from Satan), and the belief that menstruation has not ceased is just an illusion or waswas, then she should not pay attention to it and should not repeat any of the days she fasted.