It could be that the droopy position of the shoulder sensitize the upper trunk of the plexus. The sternocleidomastoid and trapezius muscles receive fiber contributions from the second, third and fourth cervical nerves. This may also affect the shoulder girdle alignment so the levator scapulae muscles gets tensed and may cause shoulder pain. We know also that the nerve has visceral connections. It could be a referral pain.

These are my thoughts….