ANSWER:

You must keep all the previous missed fasts first as those are obligatory fasts and you’re sinful if you didn’t make them up.

I have been missing fasts due to menstruation from years back.I dont remember how many farj fast i missed.Now i want to keep the fast of sacred Shawwal.Can i keep it?or do i have to fulfill all the previous pending farj fast to keep the Voluntary fasts.

Fasting | June 6, 2020 5:25 am
Previous

I’ve read that if a woman is sure that her menstruation is over before sunrise, and she intends to fast, her fast will be valid even if she delays ghusl until morning. But what if she isn’t able to do ghusl even in the morning or throughout the day because of an illness e.g. high fever? Is her fast valid? What about the prayers throughout the day?
Next

In hadith it isnt mentioned about dates 13,14,15th of lunar month to fast. So is it so that we can/should only fast on ayyam al beed or we can fast any 3 days each month?