Baker's Cyst

Symptons of a Baker's Cyst is pain and swelling behind the knee, right at the junction where the upper leg meets the lower leg. It probably feels like a little glob of Jello under your skin.  

It's a non-malignant growth that typically hits runners and tennis players. Unfortunately, there is not much you can do with this yourself. You need to see an orthopedist to have it removed.