How could i gain weight or atleast make my legs a little bit thicker?

hi im 20 years old, & i have a 23 month old little girl & im aobut 5'3 ft tall and weigh 83 lbs which is still way too little for my height... i have never been heavier than that exept for when i was pregnant and gained 30 lbs in 8 months bcs i didnt even make it to full term! & when i had the baby i had a great body size 1 jeans which is great with me and size small shirts and i had a big bcs yeah that i've always had a nice not flat like other skinny people! but now im back to my regular weight and wearring size 0 jeans again and xs shirts & dresses & i mean i dont mind being thin at all i love it plus im not nasty skinny im jst reguar skinny since im short but i do wish i had nicer legs...i even went to the dr. bcs i was worried than he was worried so i have been on vitamis, folic acid & iron for almost 3 months! & i havent gained 1 lb!!!!!.... i need help plz! and i dont want to hear ''u need to eat'' bcs trust me i do eat i eat alot but idk whats wrong bcs i dont gain weight.... if u know anything i can do to make my legs get a lil thicker plz tell me! i would love your help =D