I had a similiar issue that turned out to be a tight IT band. A tight IT band can pull the kneecap out of alignment and cause incredible pain. It got to the point where I wasn't willing to walk. Go see a massage therapist or a physical therapist. A good massage therapist can loosen the lateral quads and even work on the facia around the patelar tendon. If it's a problem in the joint capsule, they'll refer you to a PT.