I'm getting physical therapy for my rotator cuff surgery but now I have a burning pain in front of my shoulder. Did they over stretch?

Likely just muscle. Physical therapy for rotator cuff is a mainstay of therapy for this. With stretching and exercise the other muscles and tendons of the shoulder joint will become strengthened and stretched. These muscles are likely not used to this at this point and so will become sore and burn as Lactic Acid builds up in them. The best thing to do is to keep up with the pt.