Is it obligatory to repeat the prayers for the days during which I thought I was menstruating and it continued for three weeks, then prayer was interrupted for four weeks thereafter, while it was (due to) remnants from a previous pregnancy and bleeding?

If a woman sees blood after a miscarriage, it is considered istihadah (definition) (non-menstrual vaginal bleeding) if human features are not discernible in the fetus, and nifas (definition) (post-natal bleeding) if they are. If human features are not discernible in the fetus, the blood is istihadah, and ablution (wudu') is required for every prayer, unless it coincides with her regular menstrual period, in which case it is considered menstruation. If the bleeding exceeds fifteen days, she is considered mustahadah (one experiencing istihadah), and she would be required to perform ghusl (definition) (ritual bath) and pray after the expiry of her recognized menstrual period. As for abstaining from prayer under the assumption that it is menstruation, there is a difference of opinion regarding the obligation to make up these prayers, but it is more prudent to make them up.