What Causes Lower Back Pain Along With Heaviness In Pelvic Area?

I have lower back pain/soreness that gets worse as the day progresses. After a days work I feel a heaviness in my pelvic area. I had a urinary tract infection a few weeks ago and took a 7 day course of Bactrum. I had another urinalysis after the 7 days since I was still having some symptoms and it came back negative. The back soreness comes and goes.