Sounds like possibly a pec minor strain.
That will help you out.
Whaty you want to do specifically for some eccentrics for that is to stand up next to a table or chair with your shoulder depressed all the way. ONce you put some weight onto the arm but not enough to cause pain and then slowly let your shoulder girdle raise under pressure.
That should put eccentric contraction on pec minor which should help heal it up.
Have you tried doing any soft tissue work such as ART on the area at all? How about around the shoulder? Any tender spots?