I think the back pumps that MOST people complain of are exactly that BACK PUMPS. There is a difference from a pump from working out or straining the lower back and pain from the kidneys.
DBOL creates crazy pumps, and one of them is on the lower back region. It gets pretty bad when doing DL's or Squats, etc. But if the pain is all day, then it might be kidney pain.
I had insane back pumps, but it only happened when I was working out, or sitting/standing for a long time in one position.
So you can't say that the back pain IS kidney pain. I can tell you from my experience that it is back pumps. The lower back muscles get really tight.
Just my thoughts!