My advice, although humble, is to go and get it checked by a doctor. There is a variety of things that can be going on such as:
- Blood clot (deep vein thrombosis)
- Atherosclerosis (plaque buildup)
- Inflammation of the leg joints by arthritis or gout
- Infection of the bone (osteomyelitis) or skin and soft tissue (cellulitis)
- Benign tumors or cysts of the femur or tibia (osteoid osteoma).
- Malignant bone tumors (osteosarcoma, Ewing sarcoma).
You also have big nerves that run through and behind your knee cap. Sometimes if those nerve(s) get inflamed, for whatever reason, they can produce the symptoms you are having.
Either way, since nobody will be able to tell you whats going on thru the internet (from the above possible things I've mentioned), get it checked to be sure.
Hope this helps. Happy Holidays and good luck.
Just a thought...Are you sure you aren't just recovering from a 'Charlie Horse'? These can take several days to recover from.