I was going to suggest your thyroid.
A friend of mine went from a size 10 to an 18 in the last month of her pg. She tried so hard to lose it but couldn't. After a few years she got medication for her thyroid problem & with exercise got back down to a 12.
She's had her 2nd baby since then & went back up to about a 14, but has to wait til she finishes BFing to go back on the medication.
She's not too worried about it yet though.