I actually have the same pain, in my right shoulder blade.
I told my teacher about it and he said that it is not normal of pianists to get pain in that region. It didn't hurt 'that' much it just felt like it was straining a little bit.
He watched my shoulders during my lesson and found out what it was.
Whenever I reached toward the lower octaves my upper body would lean left instead of moving my whole torso. I just kind of tilt over. As a result, my right arm would stretch a little more. My right elbow ends up a little bit higher than my left elbow, in relation to the ground.
Not sure if I described that very well though. My right shoulder would be higher than my left shoulder when I play low octaves and it ended up straining a bit. Now, I move by moving my lower torso, supposedly the right way, so that the arms stay parallel.
Hope that helps some, it fixes it for me but I still have to break my old habits.
Most likely, your posture is correct when sitting at middle C.