Why am I losing weight?

I'm a very petite girl. I'm only 5'1. Before pregnancy I weigh 115. During pregnancy I went up to 130, somewhere around there maybe 140. Can't really remember. The other day I went to the doctors and I weigh 112. Only reason I am shocked is because I eat A LOT. I eat almost all day and my stomach has grew. Kid you not, I have a muffin top. I thought I was gaining weight. But to my surprise I'm losing weight. Is this normal? I do breastfeed, not sure if that has anything to do with it. I didn't ask the doctor because I totally forgot about it after we got into why I was there in the first place. I'm trying to gain weight; except I feel like all my weight is going to my stomach.