Probably your tensor fascia latae or sartorius giving you problems. Both are hip flexors and share an origin on your pelvis near the crease on the side of your hip. The sartorius runs down the inside of your thigh and the TFL runs down the IT band several inches. Of course there are a lot of potential problems here, and pain tends to radiate, but just thought I would throw those possibilities out there.
You may consider some soft tissue work (Self myofascial release/foam rolling). My IT band gets knotted to all Hell. It's really insane. Some static stretching could also be of use. Doesn't sound much like a mobility problem to me. Lack of psoas activation/strength could also be a problem if you experienced hip flexor strains.