Ice it. 20 minutes ice, 30 minutes not. Elevate as well, and think- is there anything extra-gnarly you did that may have caused it? I'd let it rest for a few days. Don't ski or do anything really knee-extreme for a few days, and see if it starts to feel better. If it does, it's working. If it doesn't improve a bit within a few days, go see a real doctor.