Hi, Welcome to Healthcare Magic Forum.
is a Congenital Urologic Condition which is the most common type of Kidney fusion anomaly and is usually found as a Incidental finding. Usually it is symptomless and in rare cases it causes abdominal pain
and distension as a result of urinary obstruction, vomiting. It is a predisposing factor for formation of Kidney stones, which may be the cause of back pain in your case along with spinal muscle spasm
, if at all stones are not present. You said you are overweight and pain radiates to your sides and legs, it may be related to Spinal Arthritis
owing to your excess weight. Intravenous pyelography CT scanning is the best initial radiologic study to determine anatomy and relative renal function. You need a thorough work up and Physical evaluation by both Orthopedist and a Urologist
Good Luck and Take Care.