What is considered to be the blood that reappears after the cessation of postpartum bleeding (nifas), then stops again? Is it nifas, menses (hayd), or irregular bleeding (istihadah)? And how is the start of menstruation calculated after nifas?

The maximum period of postnatal bleeding (nifas) is forty days, and there is no minimum limit for it. So, whenever a woman sees purity, she should perform ghusl and pray. If bleeding returns to the woman after seeing purity and after forty days have passed, it is considered menstruation, unless it exceeds the maximum duration of menstruation. If it continues for an extended period, she should only observe her regular menstrual period, then perform ghusl and pray. As for yellowish or brownish discharge after purity, it is not considered anything.