Were you doing the STD tests in accordance with the known schedules for incubation of the various STDs, so you can be sure you didn't test too early? If you don't trust the doctor you went to before, I would think a dermatologist would be a good idea since he or she should know how to test for all the STDs plus would be well trained to tell you if it's a fungus or other skin issue instead. If you get no answers (which would be surprising), you could also try a urologist.