Went to er for kidney pains, just a minor uti, kidneys working 100%, no stone.what could cause that pain then in my back?muscle, nerve?

Pyelonephritis : My guess is that your back pain is due to kidney infection (pyelonephritis) is a specific type of urinary tract infection (UTI) that my begins in your urethra or bladder and travels up into your kidneys.. If not treated properly, a kidney infection can permanently damage your kidneys or the bacteria can spread to your bloodstream and cause a life-threatening infection.