Itching in urinary tract

I'm a 46 year old male and I recently had unprotected sex where the woman I had intercourse with could discharge a lot of fluid during sex. The next day o I noticed a slight itch tingling just inside the tip of the penis and down a little bit inside.
I was worried about an STD and went to the urologist. The urine test came back negative but the doctor had started me on doxycycline for 10 days. The condition did not go away, He gave me two more medications and the condition has not gone away. I went back for a second urine test and that also came back negative for any STD's
I posted on another site a doctor suggested it may be an infection by a protozoa called trichomonas. I went for a 3rd urine test and it also came back negative.
Recently I thought it was starting to fade away and resurfaced stronger. The itch at the beginning was sporadic and was not that strong now it is more constant and stronger. it seems to be aggravated after ejaculation.
Any suggestions would be helpful. Is this an STD a UTI or nothing. It seems to coincidental to have it appear the day after having unprotected sex where this woman could ejaculate a lot of fluid! Help.
thank you
Michael