What Causes Irritation And Burning Sensation While Urinating Immediately After Sex?

I recently received a pap smear, blood test, and urine test. All resulted negative, nothing showing up in my test. A month later, immediately after sex I experienced irritation, and burning while urinating. I went to the doctor and she saw something in my vagina that she took a sample of. I'm worried because she asked if I've ever had herpes before and I have NOT. Wouldn't she have been able to find herpes in previous tests? If I had herpes, there's no way I would have experienced symptoms immediately after sex, right?