I'm gonna apologize for sounding like a smartass here, I'm sorry.
AC does not refer to a muscle, it refers to either the AcromioClavicular Ligament or AcromioClavicular joint, because the ligament attaches the Acromion to the clavicle. Muscles in that area are the attachments to the pectoralis major (clavicular division) or Supraspinatus.
Sorry for that, it's just that I was itching to use my classwork in a situation.
As for the Thread creator, You could have torn one of your rotator cuff muscles, had a shoulder separation. Is your clavicle sitting higher on the injured side than on your uninjured shoulder? If so then that could be a shoulder separation which means a sprained (aka torn) AC ligament. I don't know because I cant see it right now but otherwise I could definitely tell what happened to you bro.