What Causes Pain And Swelling In Knee After An Injury?

I was hit in the knee by a softball tonight. It now has a big red ring and inside the ring is a yellowish color. Has some swelling and is pretty painful. I was rubbing it to see if I could feel a Crack or anything and I felt an indention at the exact location where the ball hit. Is that something serious? If I bend my knee very slightly, there is not much pain. But if I straighten it or bend it, the pain is much worse. I can walk on it with a heavy limp.