+1. I get the same thing and it is my top rib locking up and putting pressure on a nerve in my shoulder. I have to be careful to stretch often and if it gets bad, I go in to the chiro for him to manipulate my shoulder. Now, I understand that chiropractors in the US are considered somewhat suspect - in Canada they are highly regulated and are part of the primary-care physician network. If you are in the US you might consider talking to an osteopath.

In my experience, these thigns do not go away on their own; resting can help a bit but sometimes makes things worse if you stop movin.