Must one repeat the Maghrib prayer and the day's fast if one felt menstrual pain before Maghrib but the blood only appeared after the prayer?

Your fasting for that day is valid as long as you did not see the blood or feel its descent before Maghrib, because the default is that an incident is attributed to its closest time, which is after Maghrib. As for the Maghrib prayer, if you strongly believe that blood exited during it due to the slight pain and your seeing the fresh blood afterwards, then the prayer is not valid and you must repeat it after purification. However, if it was a matter of doubt and not a strong belief, then your prayer is valid, and the blood is judged to have descended after the prayer. The default is the validity of the fast, and incidents are attributed to the nearest time.