I Have Pain Behind My Knee And Lower Leg For

I have pain behind my knee and lower leg for month. Today the pain is at the back of the knee and higher up on my leg instead of the lower leg. It hurts all the time. There is no swelling of my leg or ankle. Aspirin does not take away the pain. I am very active and in good health except for some arthritis in my hands. I am not overweight. I am 74 years old.