What Causes Popping Sound In Knee With Swelling?

I played foootball yesterday and took a heavy knock to the outside of the knee, after several minutes i got up and jogged it off but about 2 minutes later i twisted a slight bit and my knee gave way and i heard a little pop sound, a day later it has swollen a bit and i cant move it hardly but there was no pain from a significant area unless i moved it what is your diagnosis?