Hello,
You don't mention any hand, arm or neck pain or tightness...
so I don't think it could be thoracic outlet syndrome (TOS).
But it could be due to trigger points, they often do some nasty stuff to us.
We have trigger point info on our TOS forum, here's a sticky thread link about it.
http://neurotalk.psychcentral.com/thread125577.html
more info here too-also TOS info
http://neurotalk.psychcentral.com/post388-1.html