Is what the questioner did correct regarding the ruling on spotting that occurs after purification from menstruation, and is it considered istihadah if the white discharge (al-Qassah al-Baida) appears two or more times? Does it invalidate the fast if it occurs during the day, and is the appearance of blood on the sixteenth night considered istihadah?

The maximum period for menstruation is fifteen days. Any blood a woman sees during this period is considered menstrual blood. If she sees purity (dryness or a white discharge) after her menstrual days, she must perform ghusl (ritual bath) and pray. If the blood returns within the fifteen days, she is considered to be menstruating again, and she must perform ghusl when it stops. However, if the blood, including any intermittent cessation, exceeds fifteen days, then the woman is in a state of istihadha (non-menstrual vaginal bleeding), and the blood seen after the fifteenth day is not considered menstruation.