What is wrong with my knee?

I run cross country for my high school team, and monday we started conditioning and our coach had us run about 4 miles (3.6 because our course is a little off) and i hadn't run in about half a year (yikes!). I got this weird feeling in my knee when i came home. By the kneecap it's really swollen. It doesn't hurt, but it feels like my knee is swollen up like a balloon. Its a really annoying feeling and it kind of feels like there's fluid in my knee. If i bend my knee, you can see this really big swollen area. What can i do? It doesn't hurt if i run on it. I ran today (2.4 miles) and I didn't feel a thing. I don't think i should be running on it, but i dont want to be out of practice for too long. What is it? Should i go see my doctor? Should i ice it? elevate it? I'm not really sure. Thanks for the answers!