Lower Back Pain and Brown Vaginal Discharge

Hello, I'm 23 years old. 5'8" and 141lbs. I've been having lower back pain since January 24th. On January 29th I went to the emergency room because I had blood in my urine and an intense pain/problem while urinating. The doctors diagnosed me with a (not so surprising) UTI. I took the medication and the urinating issues were cleared up and I haven't had a problem like that sense. I met with a new doctor February 2nd for a follow up and the fact that my back was still hurting. She tried to say that I probably pulled something. She told me to buy over the counter ibprofen (which doesn't help in the least) and sent me to get x-rays that same day - which I still haven't heard the results of. It's a constant dull pain right above the buttocks - where a persons "love handles" would be. It sharpens if I sit for too long and try to stand. However, when I stand for long periods of time I'm usually fine. The only pain that occurs is when I bend over or try to sit/lay down. For the last few weeks it's been hurting on both sides equally, however, in the last few days majority of the pain is on my right side. On Friday Feb 6th, I noticed a strange brown vaginal discharge whenever I wiped. It doesn't hurt to urinate and urinating doesn't help my back pain at all - unlike when I had the uti lower back pains. BC, I'm taking YAZ, which I've been on since august - and I'm starting my placebo week Monday Feb 9th. I've always been regular, I've rarely had intense cramping or pain during menstruation. And I've never had brown discharge before my period - sometimes after...but never before. Sexually, I've just started sexual activity (for the first time my entire life) with one individual january 27th - and we've only been together twice. The UTI took place shortly after our interaction - but, then again, I hadn't peed before or after intercourse, so I'd assumed that's what made things worse. The first time we did have sex there was brief pre-ejaculation intercourse - and of course it concerns me this might be std related. My doctors office is closed on weekends, and I have a ridiculously busy life. So, the sooner I can be diagnosed the better. Since I'm new to this doctors office I was wondering what particular things I should ask to be tested for at my next appointment? Knock all the semi obvious options out of the park instead of having to schedule appointments for the next month trying to "figure it out" one by one.