Unexplainable Penile Discharge

A couple months ago I began to see small amounts of cloudy, sometimes a little yellowish, discharge coming from my penis. I experience no pain whatsoever and no burning while urination. Only mild itching on inside of penis, but it is very rare and only usually after an orgasm. It was only very small amounts at a time, but consistent enough that I went to the doctor to get tested for some sort of STI, although I knew my current partner was clean. The doctor sent me home with doxycycline, I took them, and the discharge cleared up. The tests came back negative, as i expected, but the reason for the discharge was never found out. By the end of the 7 day antibiotic treatment, Since the discharge had cleared up, I figured it would be safe to have sex again. After a week of having somewhat regular sex, the discharge came back.
I went back to the doctor and he tested for trichomoniasis, and another STI swab. Results came back negative.
My question is what are other reasons for this sort of cloudy discharge, and sometimes yellowish after orgasms? because I am at a loss for reasons after being tested on more than my liking.