I fell walking and now pain behind my knee, what is it?

Se orthopedist: There could be many reasons for pain behind the knee. A pulse would imply a possible vascular source such as an aneurysm of an artery. A knee popliteal cyst or mass may also manifest itself in the back of the knee. Other common causes include tendinitis, early arthritis/wear/tear, ligament injury or meniscus tear. Low back pain may also radiate to the back of the knee. See an orthopedist.