Help i can't gain weight?!?

I was born normal 8 lbs one ounce in the dominican republic. The milk over there is different and i couldn't assimilate any milk i was given i threw it up all the time. THere was only one milk my mom could give me and she had to get up in line at 4 in the morning ever day to get my milk and it was milk shipped in from america, the red cap gallon milk in powder. three hour wait, but then my dad brought us to america and after i tried the red cap gallon milk it's been my milk since...but the doctor told my mom it would be difficult for me to gain weight after that experience. The years fly by, exercising, healthy eating, bad eating, workouts, protein shakes, I don't know What I haven't tried lol. To the point, I had a baby, 7 lbs 4 ounces and went flat 1 month later without doing anything(little pooch unnoticeable(STRETCH MARK CITY!) got rid of most of them with Aloe vera the plant wink wink, email me if you want to know more. And ive weighed the same weight since i was 14, 92 lbs. I have size 36 b breast, and a nice tiny figure it's just the food is going no where. I will intake 2 to 3 thousand calories sometimes but gave up on it, I went to my doctor and she said i have a DANGEROUSLY high thyroid level and it'd be almost impossible to gain weight. I wear a double zero which i have to get my mom to sew on a Machine because unless i live in New York or Miami it's hard to find it. Shirts no prob. It's difficult though i don't get it. My blood is 12.9 which the doctor said is excellent, but i just won't gain weight. I'm sick of being this thin, 20 lbs more would make me very happy, please don't call me anorexic, bulimic, etc. that's not the case, i love food. It's just not going anywhere:( i'm going to be 20 in april and people are either always telling me to eat or that im lucky, but im not