Urethral burning. No STD's.

I am a 20 year old Caucasian male. About 2 months ago I started getting this tingling feeling in my urethra. At times it felt like an unbearable itch that I would try to scratch with pressure on the shaft of my penis. After a few weeks I also experienced noticeable but not unbearable burning while I urinated. Using a condom, I had sex for the first time about a month before the symptoms. I went to the doctor and got a urine and blood std test. No std's were found. There were abnormalities during physical inspection of my penis either. I was given one dose of Diflucan to treat a male yeast infection to see if that was the problem. No increased urge to urinated. I still experience burning during and briefly after urination the majority of the time of the time. Could you give me any advice?