OK during the summer I had a loss of appetite and lost about 10 pounds so I got some tests done and the didn't reall find anything. But now I've ganied my appetite back but can't seem to get the weight back. Does anyone know what could be wrong?

I can't tell you why you're not gaining weight.
But, I can make a suggestion... try drinking some Ensure or one of the other supplication drinks... not to say you'll gain weight by drinking it but, you'll be getting a good amount nutrition and vitamins . You could try eating puddings and milkshakes to help you gain.