Hi Friend
As per your description and site of pain i primarly suspect an internal impingement.the posterior Wikipedia reference-linkrotator cuff muscle is pinched betweern humeral head and glenoid labrum in the postero-superior aspect.If thats the case then i will suggest some treatment options
stretching of posterior shoulder structures
strengthening of rotator cuff and scapula stabilizers.

i will also suspect
Supraspinatus tendinitis
SLAP lesion