Could you be ore specific. Waist pain is a broad area and I wish I could offer more advice.
You have had three sonograms with no diagnosis, I would ask for further test and maybe even a second opinion because something is obviously causing you pain and it is important to find the source. Pain lets us know something is wrong and we need to look in to it and find out what it is.
Have you thought about going to the "Ask the Physician" forum and asking a Physician what your next step might be? Just a thought.