You have the tendon for your thigh coming in there so it can attach to your pelvic bones. When that tendon gets tight it pulls your thigh into your hip to make it hard to move your leg and pulls in your pelvic region as well for it's attached there. The tendons usually tighten up when there has been pain to it, the muscles reaction to pain. You have to release that tendon to get rid of the discomfort. Here is that release:
(do from a sitting position)
Place your left hand on your left leg next to your body. Place your right hand over your left shoulder and find the muscles next to your spine and firmly press on them and hold. After 30 seconds slowly lower your body forward and to the outside of your left leg, keeping your left arm fairly straight as you do. When you reach your lap remain there for another 10 seconds, release the pressure but rest there for another 30 seconds. Then reverse your hand positions and do your right side.
For best results relax your body first by taking a deep breath and exhaling then remain this relaxed.