Owwie!!

I agree with the advice "get thy butt to a Dr" if things don't improve quickly. If you are cringing and clenching your teeth, there could be more wrong than just some scrapes and bruising. You want to make sure you didn't tear a ligament or a meniscus or fracture the kneecap. There are alot of important structures in there that you don't want to mess around with if you want to keep riding.

Good Luck - Keep us posted