Negative for std's, so what was it?

I had unprotected sex with a woman and three and a half weeks later I started experiencing a very irritating discomfort in my urethra with a small amount of slightly white purulent discharge. I felt like my penis had been scratched but I couldn't determine where the pain actually was, so it must have been inside. I had an orgasm during this and as soon as it happened it felt like my penis had been rubbed with sandpaper. One thing is that it never hurt to urinate. That actually sometimes felt soothing. Anyway, I immediately got tested, and when the medical professional at the clinic saw it, she said she definitely thought I had something. Based on the visual examination alone, I got an injection of 250 mg Ceftriaxone and took 4x250 mg pills of zithromax immediately and waited for my test results. The treatment went just as described- after about 3 days the symptoms got better, and after 5 they were all gone. Then I got the results- syphilis, chlamydia and gonorrhea NEGATIVE (hiv negative too, got that within half an hour). The symptoms have been gone for about two weeks, but I am paranoid about this. I really am going overboard and I keep checking for symptoms and thinking every little NORMAL feeling could be that coming on again, but like I said, there aren't any symptoms anymore. I keep wondering what it was. I think about something like trichomoniasis? They did not test for that, and actually didn't talk to me about it but if that's what it was, why did it respond to the treatment exactly like the other conditions would. Could the results have been a false negative? Could it have been an amazingly coincidental random uti? Should I get other tests? Should I just move on and not think about it anymore? Thanks in advance for any thoughts on this!