During a yoga ex. that required extreme stretch to back of kneei felt a sudden, sharp pull behind and just above middle of knee? same pain 3 wks

Have it evaluated: It is likely a muscle pull ot tear. Of course, other post-traumatic causes are possible. See an orthopedist.

Hamstring strain: It appears that you may have strained or possibly ruptured a hamstring muscle/tendon. It is probably time to have this diagnosed. See your family doctor or a physiatrist, a non-surgical neuromuscular specialist.