Dear busymom:
it most certainly could be her kidney. I had a lower back pain which got worse and worse over months. It was worse at night. I also had right sided abdominal pain and hip pain. Turned out that I had a chronic pyelonephritis. Turns out, if you have a congenital anomoly, I had a very small right kidney with double ureters, you are a set up for this. My doctor couldn't diagnose my problem, since I had no fever or elevated white count and no stones on ct. It took a urologist to find the problem.
I never had pain over my kidney area (behind the rib cage, mid to lower back) until after my surgery. (nephrectomy--kidney removal). Kidneys are tricky organs. Actually alot of organs can do this--refer pain to other areas.