I have some burning down there?

I started to have burning after I pee starting a couple weeks ago. When it began it was just kind of uncomfortable right after I peed. Now the burning is basically all the time, but still the worst right after urination. I went to the GYN thinking I had a UTI or STD. She tested for Chlamydia, Gonorrhea, Syphilis, HIV, Hep C, pregnancy, and did a urine culture, but everything came back negative. She put me on doxycyline just to be on the safe side but I am just about done with it and the burning is just getting worse. Any ideas?