What is the ruling on praying the obligatory 'Isha prayer, and what must I do, if I performed ablution for Maghrib and my ablution remained intact until the 'Isha prayer, and after completing the obligatory 'Isha prayer and before starting the Sunnah (voluntary) prayer, I found menstruation?

There is no sin upon you if you prayed while unaware of the onset of menstruation, according to the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and His saying: "Our Lord, do not take us to task if we forget or make a mistake," and the Hadith: "My Ummah is forgiven for mistakes, forgetfulness, and what they are forced to do." However, if you are unsure about the exact time menstruation began, whether it was during the prayer or after it, then your prayer is valid, because the default assumption is that menstruation occurred after the prayer.