I would go see a doc, but I would not worry too much if I were you, I had the same thing. It can come from many things, most common, sleeping on your shoulder all night. This can cause an unusual fluid build up in the shoulder. Believe me, this is coming from the king of injuries. I would definetly see a Shoulder specialist, but not too worry, you'll be fine. I have known 7 or 8 people who have torn shoulder stuff, all felt severe pain within a day.
No worries man.