do you have any other symptoms or aches besides what you mentioned?

What kind of job do you do?
Any neck tightness or arm & hand problems with the sore shoulder?

If you have a desk or repetitive type of job, previous whiplash or other neck or head injury, I would wonder about possible Thoracic Outlet Syndrome {TOS}. Some of us get muscle spasms in the chest muscle that causes some of what you describe.
If any of those apply here is our TOS forum where you ca learn more -
http://neurotalk.psychcentral.com/forum24.html


But it also could be many other things or maybe just a odd bug..