I'm a high school distance runner, currently doing cross country. In gym class on Friday we were playing flag football and as I was running the ball an opponent ran up to me from the side, and his knee hit me where the quad joins the knee on the outside of the leg. I immediately felt the muscle contract (as if I had been dead-legged or charlie-horsed), but after jogging around for a few minutes it loosened up, still hurting a little. For the rest of the day it got worse, swelling a bit right above the knee, and feeling tight, painful to bear weight or bend it past 90degrees. Didn't run at practice, to hopefully let the injury sort itself out. When I got home I iced it and massaged the muscle in that area, hoping it was just the equivalent of a cramp or a knot in the muscle, which didn't have any noticeable effect. Saturday morning it was still swollen, we had a race but I decided not to run because of the pain and risk of further damage.
I have looked online and so far my closest find is 'knee effusion', the pictures look similar to what I've got, but most descriptions cite arthritis as a cause of effusion, as opposed to a singular trauma.
Any help would be appreciated!