I wonder if it’s due to improper scapular action on the left side. there may be some wonkiness in the way your shoulder moves back and you’ll see that with scapula as you go into the stretch. the best way to take a look would be to shoot some video from the back. if you do this, first start by standing straight up with arms at sides for a few seconds. then move towards the doorway and slowly go into the stretch. pause, then slowly pull yourself out of the stretch. we can take a look at what’s going on back there.