Haven't really had them, but from what I've read it seems like its simply a pump in your back. Pump meaning the feeling of the muscle being full of blood when you finish working out. I know SD gives me some serious, long lasting pumps all around my body. I haven't hit the higher than 10mg/day yet so I haven't had a seriously discomforting back pump yet. Don't quote me on this because I'm not exactly sure.
I can tell you that if it was your kidneys screaming for help, it'd be a SHARP pain & it would only be on the right side of your body, not your entire back.
I highly doubt it's your kidneys cause there have been guys here taking all the right supps & drinking at least a gallon of water a day & still getting the back pumps.