Heard a pop behind my knee. Sharp pain in the middle of the back of my knee.

Consult doctor. It could be ruptured muscle ,or ruptured cyst which can be confirmed by scan , please discuss with your physician .
Activity? What activity were you doing when the pop occurred? It could be a rupture of a Baker's cyst (benign cyst that occurs behind the knee) or it could be a ligament rupture which is more serious. Is the knee swollen or feel like it's going to give out? I would elevate, rest and ice the knee for the next 24 hours and if it's no better see your doctor or urgent care.