[Cloud9Fitness]

This one is tough without an examination because of how often poor scapular movements can't be seen from the front. However, inflammation of the rhomboids and scapular bursa is most likely. The scapula does not have a true joint with the back, and is controlled by the traps, rhomboids, levator scapula, and serratus anterior. If you have dominant pec and lats, you may be pulling your scapula out of preferred alignment. This may disrupt the bursa. The final shrug motion that caused pain may have been either the rhomboids insertion on the bone or the bursa right under it.

Treatment is rest until pain subsides and beginning exercises that incorporate proper scapular movement. Tons of shoulder/scapula exercise can be found all over these blogs. I would specifically suggest trying upward rows instead of shrugs. This gets the upper trap too because of the shrug at the top and the upperward rotation of the scapula needed in the movement (which is part of its "functional" role).