Pain in Hip/Upper Hamstring

I've been having what feels like a cramp or muscle spasm across the back of my left thigh. It starts in the lower ball-joint area and goes across the back of my thigh, below the glutes. It's mainly there when I'm standing still. Shaking my leg out doesn't help, shifting my weight doesn't help, walking around or sitting seems to do the most good. I've tried hamstring stretches and they help - a little, and piriformis stretches help - a little, and bending over to touch my toes to stretch my back helps a little, too, but I can't figure it out. It comes and goes but when it's there, it's really painful. Yesterday we were shopping at the LBS and talking to the owner for awhile and the pain got so bad I thought I was going to pass out.

Any ideas? My family doctor usually prescribes Skelaxin, etc., but I want to find the cause and get rid of it - not just ease the symptoms temporarily.