Having Troubles For The Past Year And A Half. Redness

Having troubles for the past year and a half. Redness on the head of the penis, surrounding the urethra. The small bumps and redness come and go in severity seemingly at random (although symptoms become very pronounced during and after masterbation). The ureter always burns to some degree, and can become very uncomfortable. I began noticing these symptoms a few days after an unprotected sexual encounter (stupidity) and had to pee extremely frequently for the first couple weeks. I've seen multiple doctors and urologists, they've taken many urine samples, taken blood, and taken a culture, and so far everything has been negative.I have taken docicyclin and tried antifungal cream to no avail, although the pills did seem to alleviate the symptoms, most notably the burning sensation. Any help wod be greatly appreciated!