Struggling to gain weight postpartum?

I have a 4 month old. I was sooo happy with how quickly I lost the baby weight, after he was born. But now I can’t stop losing weight. I started out as a size ~5~ before I had him, and now I’m a size 2! I went from 140 before I got pregnant, to 170 while pregnant, to 120 now. Anyone have any advice on how to gain some of it back (besides just eating)? I’ve literally had people ask if I’ve been eating or if I’m on something because of how much weight I dropped, nothing I own fits, and I don’t want it to just continue to get worse.