What Causes Swelling And Stiffness In Knee After An Injury?

I was playing basketball and my knee slide on the ground and i felt a pop from it. It didnt hurt but I jumped later and my knee bented. It was like the top of my knee's muscle on the left side of my kneecap got stretched or gave out. Now its stiff and swollen and its been 3 weeks. The swelling went down some but how long should I be from play ball again. And what do you think is wrong with it