Baker's cyst

A Baker's cyst, also known as a popliteal cyst, is a pocket of joint fluid that forms a lump behind the knee. Often a Baker's cyst is painless, but occasionally this pocket of fluid can tear open and drain into the tissues of the lower leg, causing pain and swelling.

Symptoms include:

Swelling behind the knee that may increase when a person stands.
Discomfort behind the knee and into the upper calf, especially when the knee is completely bent or straight.