What is this pain in my thigh?

I have a pain in my right upper thigh towards the back of the leg (on the side, but closer to the back than the front). It doesn't hurt to the touch, it doesn't hurt while walking, running, using the elliptical, or any other form of exercise, but if I put too much weight on my right leg while bending down or leaning, it is a sudden pain that I can't pinpoint. I don't know if it is a muscle, tendon, joint, or something else. It just started about 2 days ago and I have no idea why! When it happens, it hurts so badly and I can't help but to grimace and groan! Has anyone else had this problem or can tell me what is wrong with my leg? HELP! THANKS!