Do little, wait for swelling to go down. Doubt it's a bruise, but it might be.
If swelling doesn't drop in a week, see doc, of course. It could be serious stuff like cruciates inside the cap, it could be a bee sting.
If you can run and play tennis, it's probably not too serious, but could still be a structural problem, see doc.
Also, you need to locate the swelling EXACTLY, and the pain, if any exactly.