Actually the location would probably make it a thoracic aneurysm if it's an aneurysm, but it is still an emergency if it is what's going on.
I am sorry to say it is possible that this is an aneurysm. You should call 911. Don't allow them to give you aspirin as it is probably not a heart attack, I'm guessing. But do let them take you to a cardiac type hospital ASAP.
I don't mean to alarm you, but this is possible.