The knee is a synovial joint and is filled with synovial fluid. Seems like you are overdoing it and the joint swelled to protect itself. It is your bodies last line of defense in an overuse type of situation. It does this to protect the joint/ligaments. Just give it some time to heal up. You probably never felt anything pop or tear because nothing did pop or tear, which is exactly the way our bodies are designed for these types of overuse situations. I would just ice it and be nice to it haha. As for the infection...that is pretty crazy. Did you have a cut in the area or anything? That is pretty rare to get an infection in an inflamed joint without an open wound of some sort, but not impossible.