Is the Asr prayer valid if one feels wetness during the prayer, then bathes for Maghrib and discovers menstrual bleeding after the Asr prayer?

What you doubted about the discharge of moisture has no ruling, and your prayer is valid; because doubt about hadath (ritual impurity) does not nullify wudu’ (ablution) nor does it affect the validity of prayer according to the majority of scholars, who cite the Prophet’s (peace and blessings be upon him) saying: "He should not leave (his prayer) until he hears a sound or perceives a smell." This hadith is a great jurisprudential rule that things are presumed to remain in their original state until the contrary is certain. We advise you not to surrender to obsessive whispers, and you must seek refuge in Allah and supplicate to Him to remove this ailment from you.