Why does it burn when I pee?

I have had problems with recurrent UTIs for a couple of years now and frequent urination persists even when the infection has cleared, yet I don't have pain. I have not had problems for a while but I recently have started having sex again (for the first time in a while) and I had another UTI I assumed as I was in pain and there was blood in my urine. I took a course of trimetheprim and the symptoms reduced somewhat but not altogether. I had a urine test done but there is no infection, yet it burns when I pee and I have to go lots (even when sleeping). I contracted chlamydia 6 months ago and it was treated. Is it possible I still have it? It's the only explanation I can think of. I really like my new partner and I'm terrified I've given him something :(