Well, one of the causes of pain in the back of the knee can be called a "Baker's cyst", which often causes pain in the back of the knee that worsens with straightening it out, or when you're walking and using the knee.http://www.mayoclinic.com/health/bakers-cyst/DS00448
Of course, usually a cyst can be felt in the space behind the knee.
Other causes can be tendon inflammation in that area.
The fact that it occasionally travels up and down could also mean it is a sign of nerve irritation, or part of the sciatic nerve, which can be irritated anywhere along its course starting from the lower back, into the buttock, the back of the thigh, the back of the knee, into the calf.
Occasionally pain from the front of the knee (kneecap) can be felt as though it is coming from the back of the knee.
Given the fact that the pain getting worse, and in a different location than you are used to, this is worth having a doctor examine the knee and order further testing if he/she feels it is necessary.