i've been there too. just wanted to offer my sympathy for the comments like "i hate you, you just had a baby and you're so skinny!" um, rude! how would someone like it if you said "i hate you, you just had a baby and you still look 7 months pregnant!" to me it felt like the same sort of comment. I really hate it when people talk about my body. especially because you can't politely respond "actually i'm too skinny and i'm trying to gain weight" because they'll hate you even more. i felt like "you look GREAT" was an insult, and really got sick of hearing it. (i'm sure it was meant as a compliment, but i just hated it) luckily people stop saying that once it's been long enough, and comment on your baby instead. that was a nice change. although now i'm so tired of hearing "she looks just like you!"... ;)
anyway, i just started eating more. like 3 helpings at dinner, and snacks like yogurt and whole milk. it worked so well that i went on the first diet in my life a few months ago to lose the chub on my thighs :) in any case, the most important thing is that you feel healthy and eat when you're hungry.
another possibility you may not want to know about... i know someone who was super skinny, and it turned out she had tapeworms (parasites)... gross! hope it's not that!