What all may be causing my knee stiffness and swelling?

Many causes. Septic arthritis, rheumatoid, wrighters syndrome, ligament or meniscus tear, joint effusion, osteoarthritis, lupus, etc.

Related Questions

23 y/o w/ chronic swelling of the knee & stiffness after marathon training. Clean MRI and blood work except 1:320 ANA titer. No suspected ra. Ideas?

Joint fluid analysis. Although it's possible that this is related to your marathon training you need to have a joint fluid analysis to see if it's inflammatory or not inflammatory, if it's infection or not infection and to see if there are crystals or no crystals. You need a evaluation to make certain what is going on inside the joint. An MRI is not sufficient.