Did you injure this while running? this is usually found in runners but not unheard of in cyclists/triathletes.

if you've not had a bike fit, check seat height - posterior knee pain is a result of saddle too high. If your seat height is not out of range, for recovery and rehab you can drop the saddle height 2-3mm. Be sure to adjust saddle fore/aft to accomodate (as you lower, you bring saddle nose to bar; when you raise the saddle, it moves away from the bar on the seattube angle plane)

also, excessive float in the cleat can exacerbate injury. have a fitter check your cleat alignment, and try moving the cleats further back.

check with your PT about rehab, but here is a video link of popliteus muscle rehab exercise: http://www.youtube.com/watch?v=ViFXQra8GZw