What Causes Recurring UTI With Right Sided Lower Back Pain?

I have been experiencing back to back UTI s for over a year now. It recently went away and my urine did get sent out to the culture and nothing seems to show that I have UTI s again . I am currently taking Theracran HP supplement, but I am starting to have lower right back pain out of nowhere. Usually my first sign for UTI would be the back pain. I was tested positive for LEU and negative for NIT (using AZO strip test). What can be the problem, is it because I did not have a full bladder before urinating or there is other issues?