I'm a 16 yr old girl and I weigh 112.6 lbs. I can't gain any weight over 120lbs, I haven't been able to for 3-5 yrs. I have belly fat that I want to lose but I can't figure out how to lose it. I've also been having sharp pains from my lower abdominal to my upper chest for 4yrs. What would cause the pain? My doctors can't find anything wrong. Please help.