The first thing that comes to mind is a Bakers Cyst (popliteal cyst), named after a British surgeon William Baker. What happens is excess synovial fluid from the knee joint is forced to the back of the knee. This bulge is referred as a Bakers cyst. There are many different approaches to fix this: Medication, Aspiration of the joint, Arthroscopic surgery and Cortisone injection.
It would not show up on x-ray, but would on a MRI (Magnetic Resonance Imaging). Hope this helps! jd1963