Hi,
I would see your GP as well as a nutritionist. I am in the same boat and had to have various medical tests to rule out medical conditions. Everything was ruled out. I then had to keep a food diary to give my nutritionist an idea of what I was eating in a day, but that was all normal too.

Now I am currently working on building my muscle tone to fill myself out a bit and give me the extra weight. Nothing I did or could have done would have made me put weight on and maintain it, so this is my only option.

Don't worry about turning into a body builder that takes a lot of work and protein and special body building exercises. I do mainly dancing and calisthenics to build up my abdominal, gluteal (buttock) and quadricep (thigh) muscles.

The thing I found the hardest were the comments other people made about my lack of weight, and the fact it seems to be socially acceptable to make comments about being too skinny but not too fat. It hurts.

Good luck!
I didn't see your answer before
What I always get is "I wish I was as skinny as you" And things along those lines, But I don't like it It's not fun for me.
I have had people try and accuse me of being anorexic or bulimic but they soon stopped accusing when they saw how much/often I ate and the fact I didn't run off to the loo after
I just want to look/feel healthy and have clothes fit me properly!! I float between sizes and depending on the brand I have to even start looking in the kids section(the largest size) for pants that wont fall off my non-existent hips