She did not fast because she was worried about her nursing baby
A woman gave birth in Ramadaan and did not make up her missed fasts after Ramadaan because she was worried about her nursing baby. Then she got pregnant and gave birth in the following Ramadaan. Is it permissible for her to give charity instead of fasting?
Praise be to Allaah.
What this woman has to do is to fast and make up the days she has missed, even if it is after the second Ramadaan, because she had an excuse not to make up the days she had missed between the first Ramadaan and the second. I wonder whether it will be too difficult for her to fast day after day in the winter even though she is still breastfeeding, but probably Allaah will give her the strength to make up the second Ramadaan. If she is not able to do that then it is OK for her to delay making it up until the second Ramadaan.
Fataawa al-Shaykh Ibn U'themeen