Penis - burning sensation mainly after ejaculation

Hoping someone can assist me on my issue here, at its becoming a bigger concern. I am a 27 year old male. I had unprotected sex with a female around the beginning of June and shortly after I noticed a mild burning sensation in penis tip when I ejaculated. I knew this was likely to be some kind of STD as a result, (I had been diagnosed and treated for non-specific urethritus 2 years earlier) so proceeding to get an appointment with my GP. He prescribed a 10 day course of Doxycycline 100mg capsules which I took as prescribed. I returned to the doctors as my symptoms persisted. I gave a urine sample (which was tested for such infections) and also the penis tip swab test which all came back “clear”. However, I explained that my symptoms were still present and felt the need for a longer course of antibiotics, and my GP pointed out that urine tests are not always 100% accurate. He prescribed a 14 days course of Erythromycin 250mg which I also completed, and still the symptoms persist. To try and clarify these symptoms further, it’s difficult to find the best adjective to describe, but I guess it’s a burning or mild stinging sensation that is most prominent after ejaculation, but I can also sense it at other times of the day, noticeably more in the evening perhaps. Its not a massive or unbearable pain, but something I know should not be present and as a result is becoming more of a concern for me, especially after the antibiotics prescribed. Based on this, any advice on the would be greatly appreciated: Could this still be an STD that was undetected on the urine test and swab test AND resistant to the medication outlined above?? If so, is there any other way of finding out? Also, having read similar cases to this on-line, a lot of people are recommended to visit a urologist. Would this be beneficial based on my symptoms and what tests would they be likely to carry out? Many thanks in advance for any information/advice. Much appreciated.