What Causes Popping Of Knee With Severe Pain?

I play tennis, and have for a long time. recently I was runing laterally and my knee popped - seemed to go out of joint a bit and hurt. When I just walked after the incident it didn't hurt at all, but if I tried to run the knee popped again and bothered me. Its been three weeks, and now I can run laterally, but if I go for a drop shot my knee does the same thing. ???