I've suffered from Body Dysmorphia since I was a girl. I understand completely.
You are at a healthy weight, and since you are asking "what's wrong with me?" I'm guessing you already know that you are in fact not overweight.
First...I would talk about it with your doctor. That being said, doctors are often ill-equipped for dealing with issues like this. So, be prepared for little or no help from them. I've felt frustration often when it comes to the lack of training doctors have when it comes to these sorts of things.
Second...If I were you, I'd ask myself what it is exactly that I'm seeing that makes me think I'm fat. In my case, I see my thighs and I groan. They truly disgust me. When I see old pictures of myself, when I was thinner, I see my thighs and I think they weren't so bad, but back when the photo was taken, I thought my thighs were horrifying.
So, locate what it is that you think is horrible. Make a list if you have to.
Third...Ask yourself what you idea of "healthy" or "thin" is. Those might be two different things. If you have an image in your mind of those words, ask yourself where those images came from. Often we carry around moments in our lives that cement unreal expectations into our consciousness. You need to confront those images. You need to question them.
Lastly...as someone said before, it might just be an issue of body composition. Being more muscular might help you feel better about yourself. Obsessing about a number on a scale can often be overcome by building muscle and FEELING healthy. So, work on strength training.
| current weight: 138.0