Why do my knees pop every time I extend my legs?

I am a soccer player. Season ended last week. I pulled a tendon in my left knee a few weeks ago and pain was minimal. I also slammed into another player in a game with my other knee and there was bruising and pain. Now whenever I run there is pain in both knees and whenever I extend my legs,they pop(it's audible) and there it is painful. I tried working out on a stationary bike but it hurt too badly. What is wrong?