RICE is the key to life right now. Without any other information I don't have much advice for you. Did you land directly on your knee/knee cap area?

Is there a lot of bruising? If there is you really should have an X-ray.

How long did it take for the swelling to show up? If it became swollen in less than a couple of hours....X-ray.

Are you having trouble weight bearing? If so...X-ray.

I'm sensing a recurrent theme here.