What should I do if my menstrual period began after the 'Asr prayer and I only became aware of it at the time of the Maghrib prayer? Is the Maghrib prayer obligatory for me?

If menstruation began during the prayer time, and there remained enough time for a complete rak'ah (unit of prayer), then that prayer does not need to be made up (qada'). This is based on the Prophet, peace and blessings be upon him, who said: "Whoever catches a rak'ah of Fajr prayer before the sun rises, he has caught Fajr. And whoever catches a rak'ah of Asr prayer before the sun sets, he has caught Asr." This is the view of the Malikis, because that by which a prayer is considered caught and becomes obligatory is also that by which its obligation is lifted.