Is it permissible to feed the poor in lieu of the missed fasts during menstruation in previous years, which I was unaware needed to be made up, especially since I am not physically able to fast them now? And if it is permissible, how should the feeding be done given that the number of days is unknown?

A woman must make up the days she missed fasting due to menstruation. This make-up fasting should be completed before the next Ramadan, unless there is a valid excuse. If she is unable to make up the fasts due to an illness from which recovery is not expected, she should feed a poor person for each day. If she fasted during her menstrual period out of ignorance, she must make up those days. If she did not fast out of shyness, she must also make them up. If she has accumulated many days to make up and is unable to fast due to old age or illness, she must feed a poor person for each day after accurately calculating the number of days.