How can one differentiate between menstrual blood and istiḥāḍah (non-menstrual vaginal bleeding), especially if the bleeding is accompanied by light discharge and menstrual pains, and what is considered light bleeding and discharge that appears intermittently after inserting an IUD?

The basic principle is that any blood a woman sees is menstrual blood. If the bleeding stops, the minimum period of purity between two menstruations is thirteen days. If she sees blood after this period, it is a new menstruation. However, if she sees blood before the passage of thirteen days, and it is possible to combine it with the previous blood to form a single menstruation (meaning their combined duration, including any intervening purity, does not exceed fifteen days), then it is considered a single menstruation. If it is impossible to combine it, and it cannot be considered a new menstruation, then it is Istihada (non-menstrual vaginal bleeding). Therefore, any blood seen after sixteen or fourteen days from the cessation of bleeding is considered a new menstruation.