What Causes Swelling And Pain In Knee After An Injury?

I went skating friday night.. and the first thing i did was turn around to go backwards.. and my knee completly collapsed.. Now its super swolllenn and it hurts really bad.. On the outside of my left knee it looks like a softball sized knot almost is on it and its hard to bend and straigten all the way.. I can still walk on it but with a pretty bad limp. What is wrong?? and what can help me??