There are many reasons for pain behind the knee. Your hamstring and gastroc both cross over the knee, so one of these muscles could be irritated. You could have a Baker’s cyst (benign), or have a lymphatic backup. One other possibility is an irritation of the popliteus.
Read MoreA baker’s cyst, or popliteal cyst, is a collection of fluid behind the knee. It is benign, and often asymptomatic. If it gets severe enough, it can cause pressure on the local nerves and vasculature which will provoke pain and stiffness in the knee. It will usually be difficult to fully flex the knee.
