Have I committed a sin by seeing my menstrual period after returning from the 'Ishā and Tarāwīh prayers at the mosque, without being aware of its onset?

There is no sin upon you if menstrual blood flows while you are in the mosque or during prayer, because it is beyond your control. However, you must stop praying and leave the mosque. Its flow after all of that is nothing to worry about, for menstruation is something Allah has decreed for the daughters of Adam.