Dear patient Back pain with uti may be due to pelvic inflammatory disease which is inflammation of pelvic organs. But since u out have pain going into both legs possibility of disc prolapse or canal stenosis is more likely and seems a different condition from uti. Is there any difficulties in walking or tingling and numbness in both the legs? If yes then it also points towards canal stenosis. You need to be investigated with mri of lumbosacral spine with screening of whole spine as it gives detail information of discs , nerves and even bone. Visit radiology center nearby you and get it done. Visit spine surgeon with report. Meanwhile start tab diclofenac plus thiocolchicoside combination twice a day for pain relief plus tab vita B12 plus pregabalin 75mg at bedtime for 10 days. Also get examined and search for the cause of recurrent uti.