I'm only 103 pounds and I'm 5 foot 7 inches

Don't be self conscious, you are very beautiful. If you want to gain try drinking Ensure or meal replacement drinks in addition to meals (several times a day). This protein will help it be a healthy gain and not just fatty. It's all calories in vs calories out for weight loss and weight gain. You just have to consume a lot more calories 

Lift heavy weights. Eat at a calorie surplus, aim for high protien foods.This will help you add muscle that will keep a firm tone look while gaining weight.

Honestly try eating more and protien protien protien and ensure shakes lots of them. My niece is just like you she also went to a doctor and they couldn't find anything wrong with her they were going to give her meds to gain weight but didn't because she has another problem that would interfere with the meds so....

I am like you I was a hundred and four pounds and I am 5 foot 2 and I kept losing weight. I was at 170lbs on my wedding day. 11.11.11.. and starting losing weight after.. within 6 months i was down to 115. ( on a side note I was not dieting . I was very worried as to why I was losing so much weight at such a rapid rate) day to day stressors and working nonstop can only explain what the difference was. Back in December of 2014 I went on a trip to Mexico for just a few days kind of left reality of my life here in the States and wouldn't you know it now I'm finally proud to say I have gained 15 pounds in 3 months I strongly believe that for me at least it was the relaxation. eating tons of resort food nonstop that helped me reset and focus on me. Daily I make an effort to eat a little larger of a portion make sure I'm eating 3 meals a day and find time to relax I don't know if this will help you but I do understand. I don't know how many times I was asked what was wrong with me am I sick am i anorexic it was worse than when I was actually overweight. how people treated me for being underweight.

Everyone deserves to feel beautiful in their own hearts. My suggestion although it might seem silly but junk food might help? You could possibly just have a very high metabolism which means that your body breaks down the food super fast. In which case have you ever been to a nutritionist? Maybe they could help

This is me about 3 months ago! 

As you can see I'm very tiny 😞

Try adding some healthy calorie dense foods like nuts, nut butters, and avocado. 

My friend is underweight and her parents are both doctors. They've always made her eat frequent, high calorie snacks like nuts, cheese, or a meal replacement shake like slim fast or ensure. And then she eats normal meals. 

Protein!!!