Gaining Weight vs Losing Weight?

I would talk to a doc about it if your to concerned but Im very thin and have high metabolism (ex. I just had a baby 2 wks ago right b4 I gave birth I weighed 144 now I way 120) I lose quickly. The only way to gain is to eat like a football player. Lots of carbs and red meats. Muscle does weigh more than fat though so if you were inactive for a while you may have lost muscle m