Flank area pain is not always kidney related. You have alot of tendons, muscles, etc. that wrap that area and go from the lower back to the ribs. I had the same issue awhile back and it was a bruised muscle/tendon. Actually some of the worst pain I've ever had.
Is the pain pretty constant but get's sharp or worse at certain times??? ..or will you feel totally fine and then all of sudden, wham. pain!??