My knee is very swollen and is red and warm to the touch I don't recall of any knock or fall what could it be?

Possibly infection. Swelling and redness are usually indications of inflammation and/or infection. Arthritis, septic joint, osteomyelitis (infection in bone) can be caused. Should see physician sooner than later for examination , blood test, and possibly imaging. Some infections are virulent enough to destroy bone, cartilage, muscles etc.