Also, check out the top of this website. It could also be due to a muscle imbalance in your quads.
http://www.thestretchinghandbook.com/archives/patellofemoral-pain.php
Symptoms - Pain at the front of the knee that gets gradually worse.
Causes - Your knee cap (patella) isn't tracking correctly. This is usually because of muscular imbalances on the inside of your quads, or tightness on the outside of your quads.
Treatment - Rest is very important. While resting massage will help loosen up the outer muscles of your quad. Stretches will also help to loosen the outer muscles and work on correct the imbalances in the inner muscles.