Fasting is an obligation for every Muslim, but Allah does not burden anyone beyond their ability. If you missed Ramadan due to pregnancy and then continued breastfeeding, you are required to make up the fasts for those two Ramadans when you are able.
If you are still unable to fast due to illness, weakness, or a permanent health condition that prevents you from making up the missed fasts, then you are permitted to feed a poor person (fidya) for each missed day. However, if you regain the ability to fast, you must make up the missed days.
May Allah accept your intentions and make it easy for you.