What is the legal (Sharia) and medical opinion on the discharge of blood accompanied by pain after marital intimacy on the fifth day of the menstrual cycle, given that the bleeding had stopped and the wife thought her period had ended?

If intercourse occurred after the cessation of menstruation and performing ghusl (ritual bath) on the fifth day, and then blood appeared afterwards, there is no sin in that because it happened during a period that was believed to be a state of purity. This blood that appeared is considered menstruation if it falls within the usual period, as the menstrual period may increase or decrease. However, if it continues for longer than the usual period, it has other rulings. To clarify the signs of purity from menstruation, one should consult specialized fatwas. As for matters related to this issue medically, one should refer to doctors.