Pain Behind Knee What Can Cause Pain Behind Your Knee Cap?

What can cause pain behind your knee cap? - pain behind knee

Two weeks ago I started to notice that using my left leg to walk up the stairs hurts. As it has deteriorated. Now is when I go out of your chair at work, I can not load, I have my right leg, so that with my car. Now I'm sitting here, my knee is throbbing and you feel the pain behind his right knee. What is possible? I had cracking problems with the knee in the past often ... Suggestions? Thank you!