You might want to look at some of the images on this site:
http://www.painclinic.org/musclepain-backhipbuttockleg.htmThey might be able to help you pinpoint the cause and provide some suggestions on what to do about it. It *could* be the Piriformis muscle becoming too tight and needing stretched, which I dealt with very quickly and effectively with stretches to pinpoint it. Then again, it might not be. But I think some of the images might help you figure out exactly what muscle is giving you the problem.