When is the onset of menstruation considered to have begun: Is it from the day blood appeared due to an IUD, or from the day contraceptive pills were stopped, knowing that the bleeding during the period of taking the pills was caused by them?

The default assumption for blood seen by a woman is that it is menstrual blood. It is only considered istihadha (non-menstrual bleeding) if it cannot be considered menstruation.

So, if a woman sees blood, she considers herself menstruating from the moment she sees it.

If the bleeding stops before fifteen days, all of it is considered menstruation.

If the bleeding continues and exceeds fifteen days, it becomes clear that she is experiencing istihadha. In this case, she refers back to her previous cycle and considers that period as menstruation, and anything beyond it as istihadha.

If she does not have a known cycle, but can distinguish menstrual blood by its characteristics (color, smell, consistency, pain), then what she identifies as having the characteristics of menstrual blood is considered menstruation, provided it is suitable for that classification, and anything beyond it is istihadha.

If she has neither a regular cycle nor the ability to distinguish, then she considers six or seven days as menstruation based on her best judgment, and anything beyond that as istihadha. She performs ghusl (ritual purification bath) after the days of menstruation, then performs wudu (ablution) for each prayer after its time has begun, while taking precautions by tying a cloth. She then prays the obligatory and supererogatory prayers until the time expires. She is subject to the rulings of ritually pure women until the next menstrual period arrives.