My right knee used to feel like a stake was being driven through both sides when I walked down stairs last summer. I did rehab work for it, turns out it was all the sports I have been playing my whole life. Had to do with the muscle not properly growing due to constant wear on it. I did not think that was possible but what ever.

Anyway, they had me stand and put your heels on an incline so you are leaning forward. Then without any weights do squats with your legs shoulder width apart. I did 3 sets of 10 then I would do this again with my feet level. Give that a shot and see if it helps. On the squat come all the way down so your in a seated position then slowly stand back up. Dont rush through them.