What Causes Sharp Pain Outside The Knee Radiating Down The Calf?

I m experiencing sharp pain on the outside of my right knee. The pain travels around my calf and down the back side of my calf. Having a hard time bending my leg or climbing stairs. It almost feels like I will lose stability in my knee while trying to stand from a sitting position. Is this a strain or something worse? It came on suddenly and not sure what I did prior to the onset of pain.