Is it obligatory to repeat the Asr, Maghrib, and Isha prayers if the impurity of the undergarments is ascertained after their completion? And should the Fajr prayer be repeated if the questioner felt the pain associated with blood before the prayer but did not see the blood until shortly after the prayer?

If the brownish discharge (Kudrah) appears before menstruation and is connected to it in terms of its timing, it is considered menstruation, which prohibits prayer and fasting. So, if it appears before Asr prayer during the usual period or is connected to it, one must stop praying and subsequent prayers until purity is achieved, and there is no make-up prayer (Qada') for you. However, if the brownish discharge appears outside the time of menstruation, it is not considered menstruation, and the prayers performed without purity must be repeated, because the brownish discharge is impure and invalidates ablution. And if the brownish discharge extends until the dawn of the next day and is connected to menstruation, then all of it is considered menstruation, and there is no make-up prayer (Qada') for you.