My knee is completely swollen, but I don't remember injuring it. What is causing this?
You'll need to see an Orthopedist to confirm the cause of your swelling. In general though, swelling with no clear cause may be a result of a ligament sprain, damage to the cartilage in your knee, damage to the Meniscus. In any case, try to avoid putting weight on the knee, and start icing it to reduce the swelling until you can see a doctor.