What Causes Burning Urination,nausea And Back Pain?

Hi. I am 19 and have been experiencing symptoms of a UTI for the past week (buring during urination, frequent urination, pressure in abdomen, nausea). Also my period came very light and lasted for a day. However I sometimes skip when on the pill. This happened after I had had sex a week ago (almost a day after). We used protection and I have been on the pill for 2 years. My physician said he believes it may be something with my pelvic area. My back hurts (lower/mid) and the pain gets worse throughout the day and increases when I sit down. While standing the pain is not very bad. What can this be?