Can’t Make up Missed Fasts Due to Menses: What To Do?
Question

I did not fast on the days of my period in previous years, and I did not know that I had to fast these days before the next Ramadan came. Now I suffer from medical problems and I am weak, and I cannot fast these days. Is it permissible for me to feed poor people instead? If that is permissible, I do not know how many days are involved so how should I go about feeding them?
Praise be to Allah.Making up missed fasts due to menstruation
When a woman does not fast because of her period, she has to make up the days that she missed , because ‘Aishah (may Allah be pleased with her) said: “That – meaning menstruation – used to happen to us and we were told to make up the fasts but we were not told to make up the prayers.” (Narrated by Muslim, 335) 
Delaying making up missed fasts
We have already stated in another answer that it is obligatory to make up days missed in Ramadan before the next Ramadan comes ; it is not permissible to delay it except for those who have a valid reason. 
Fidyah for missed fasts
Whoever is obliged to make up missed fasts but then is unable to do so because of sickness or weakness for which there is no hope of a cure should feed the poor instead, one poor person for each day.
 Shaykh Ibn ‘Uthaymin was asked in Fatawa Arkan al-Islam (p. 455): 
A young girl got her period and she fasted the days of her period out of ignorance. What does she have to do? 
He replied: 
She has to make up the days that she fasted during her period, because fasts observed whilst menstruating are not accepted and are not valid, even if she was ignorant of that, and there is no limit to the time when they can be made up. 
Here is a case which involves the opposite scenario: A woman started her periods when she was young, and she was too shy to tell her family, and she did not fast Ramadan. This woman has to make up the month that she did not fast, because once a woman starts her periods she becomes mukallafah (accountable for her deeds), because menstruation is one of the signs of having reached adolescence.”  
He was also asked about a woman who did not make up the days from her period in Ramadan until she owed nearly two hundred days, and now she is sick and old and cannot fast – what should she do? 
He replied: 
“If this woman will be harmed by fasting as mentioned in the question, because she is elderly and sick, then she should feed one poor person for each day. She should calculate how many days she owes from the past and feed one poor person for each day.” (Fatawa al-Siyam, p. 121) 
For information on how much must be given when feeding the poor, please see question no. 38867 . 
In conclusion, if you are able to fast, then you have to make up the days. If you will not be able to fast, then you have to feed one poor person for each day. Try to figure out how many days you did not fast until you think it most likely that you have reached the right number.
And Allah knows best.