Had rotator cuff surgery and can't do a stretch HELP!!?

I had a bone spur removed and a small 90% tear repaired on my rotator cuff about 4 weeks ago. I've been going to physical therapy and when I do my stretches it pinches and hurts a lot on the inside (towards the chest) of my shoulder joint though. Is this normal? I still feel sharp pains when I'm stretching in different ways during my exercises. One exercise that really worries me is the hands behind the back exercise. I can't move my hands up my back without my injured shoulder moving up. I keep trying to keep my shoulder down when doing this stretch, but it keeps coming up and I hunch over...it's extremely painful and difficult to do. When I keep my shoulders down and just bring my hand together I can feel the pain and stiffness without even bringing my hands up my back. Is this normal after 4 weeks? Also is there a website that shows where you should be and what stretches should not hurt by certain time frames of physical therapy? I want to know if I'm on track or if there is a problem. I've been really upset about this.