In reply to RobertHepburn:
I'm afraid the answer is gentle mobilisation until it settles down, can take a couple of weeks and it sounds like you haven't really given it a proper rest, it's always difficult to remote diagnose but it sounds like you have pulled some of the muscles that insert on the medial border of the shoulder blade (rhomboids/levator scapulae/serratus anterior)
The best way to stretch them is to give yourself a hug!
Get well soon!
Mike