Just a thought. I was wondering how much exercise you do. There is a great exercise for your lower back. You lie on your stomach and clasp your hands behind your head. Lift your legs and your head and torso at the same time so that only your stomach is touching the ground. Do this 10 times if you can or work up to it. Try to do this every other day if you can and incorporate it into your exercise regimen (I hope you have one).
This is in case it's not a kidney pain of course. I hope this helps.