tali i have always been a thin/skinny girl and i know what you mean about knowing that you were too thin but feeling weird when you do actually put on weight
im 170cm and at the moment about 49kg. last year i was 56kg and to me i thought i looked pg because i actually had a belly that i never had before.
i think sometimes its hard for us naturally thin gals because everyone else thinks that we have nothing to worry about. i know i have just as many body issues as overweight girls and i shouldn't be judged for feeling that way.
just remember that you are what you are and you are beautiful for being that way.