What did I do to my knee?

Yesterday at my basketball game I drove to the basket and I tripped over the girls foot with my right leg, but as I did so my left knee gave out and my knee hit the ground hard. I didn't play the rest of the game. I was able to extend it but it still hurt. This morning I was able to walk around on it but it was stiff. When I press on the inside of my knee it hurts and I can extend it but I feel some pain and stiffness. There is a bruise there today (not where my knee hurts though) and a little swelling. Did I just badly bruise my knee or could this be an acl tear?