What do these symptoms sound like?

On and off for a year and a half now I been getting these symptoms where I get dizzy,lightheaded,headache and short of breath.I just don't feel like myself.I been to the ER numerous times I been to every doc possible and head so many test and they found nothing.What could it be????I am 19.I been checked for diabetes and I got a number of cardiac test and the only thing it revealed was pvc's which were harmless and probalby from stressing over these symptoms.I got 2 cts and a mri of the brain.Oh yaeh and I am not pregnant!!!