Testing while on doxy can give you false negatives, and even testing 5 days after stopping might, too. Did a doctor prescribe the doxy, and what for?
Unprotected oral can give you something called NGU, which is an infection in the urethra called non-gonococcal urethritis. It's basically any infection not caused by gonorrhea. It can be caused by normal mouth bacteria entering the urethra, strep, respiratory viruses, etc.
Did the urologist send your urine off for a culture, or just do STD testing? Again, though, even if that happened, because you were on doxy at the time, it might result in false negatives.
Unprotected oral can also transmit gonorrhea and chlamydia. The doxy would have cured chlamydia, if you took it at the proper dosage. What did you take and for how long?
To get accurate testing, you need to be off the antibiotics for at least 2-3 weeks.