just can't gain weight.

I've always been just super skinny, even during both of my pregnancies I had a tiny bump and just didnt weight much either and just gained a few pounds. I was healthy, the babies were healthy as well though my first was born happy and alive but my second one passed away at 32 weeks. I still feel like its my fault- I wasnt underweight and the doctors did tests and said it wasnt my fault and that I and my placenta and everything was just perfectly fine.