Dan, In my experience, this is rare. It COULD be that you are letting the bow shoulder ride up either as you lift the bow or during the draw. IF the upper ball of the humerus isn't seating into the socket of the shoulder, you MAY be experiencing a slight impingement. Theses are just guesses, would have to see what you are doing, video time!!
The rotational draw (NTS) advocates a static bow shoulder and arm, there should be no movement of the arm left or right related to the shoulder.
Arne