What Leads To Sharp Pain In Knee,back And Foot?

I am a healthy, active 70 year old woman.I first started having a pain in the back of the knee a bit to the right last night. When I make some movements (not all), it is very sharp, such as, if I was lying on my right side and I lift the leg, it sends a sharp pain in the area, or if I'm sitting the the driver's side of the car and go to get out, it does the same thing. It also gives me some discomfort in my foot and up the back of my posterior.I don't remember injuring it or pulling any muscles.