Sharp pain/stiffness between scapulae after rot. cuff exercise

Hi,

I started doing rotator cuff exercises three weeks ago (internal, external, elbows down and to the sides (four exercises)), because of left shoulder pain when passively protracting left scapula. I also began keeping scapulae strictly retracted and depressed in all exercises (since the pain was felt only when i let scapulae protract/elevate). The shoulder pain is gone, and I can do passive protraction/elevation without pain now.

When I began the new exercises/keeping scapulae strictly depressed/retracted, my upper back, mostly between scapulae, got really stiff and I feel i sharp pain in some circumstances.
I do full body workouts every third day, but the pain/stiffness is felt every day. The worst pain is felt when I've eaten a lot, lied on my back and then sit up, extending my back and retract and depress scapulae. This I can't do the first time i try since the pain is to sharp. After about 30-60s and a couple of tries it's possible.

What induces the pain: Full stomach, half sitting/lying on back, rotating torso, retracting and depressing scapulae, deep breaths when I haven't taken any for at least a minute (pain starts when about the last 30% of VC is to be inhaled), hiccups (this really hurts). Extending spine, retracting and depressing one scapula and bending and twisting torso on the same side is the most painful.

I didn't feel any thing like this before i started my shoulder rehab program. I do suspect that i depress/retract scapulae to hard. I really do tuck them in as hard as i can. Could that possibly be the cause?
If it is, I would like to know what happens when i retract/depress them to hard (just because I'm interested).

Has anybody experienced anything like this?

How do I know how much retraction/depression is sufficient?