Not baby related.

So I’ve always been a petite person, after giving birth to my baby boy my weight was at 120lbs I felt great and it stayed that way for 6 months. Now I’ve been loosing all that weight and I am back to 100lbs and I feel skinny and disgusting. I’ve reached out to my doctor and all she told me to do was to eat frequent meals but that isn’t working for me.. anybody knows any other ways I could gain the weight again?