M.C.
I have had times when I really pull a muscle near my ribs (or my rib goes out of place) and it causes major issues in my shoulder/back. The chiropractor explained that it was because my body was automatically adjusting my posture and such due to the muscles, and the way everything in that general area is so connected that it can really be painful in unexpected areas. The area where the pulled muscle actually was never hurt me... But I did have the deep pain in the shoulder blade, down the arm, and under breast area, as well as the "pulsing" sensation and the changes when breathing. The only big difference between what you describe and my case is that I KNEW I pulled the muscle when it happened, and I knew the pain was a direct result of what I was doing at the time... It didn't just randomly start. Since the first injury, it does come back occasionally, but I get it often enough that I can recognize it for what it is. It typically goes away on its own, lasting anywhere from a few hours to 5-6 days... Though, it usually only lasts a day or so.
I can never actually do anything for it, other than chiropractor visits (which I really can't afford... Lol.) sometimes, a heat pad helps a little. I tend to stretch where I link my arms overhead, and bend side to side... Sometimes it helps a little, but sometimes makes it worse (so try that at your own risk. Lol.)
I would really reccomend getting it checked out though... Maybe try a chiropractor instead of the ER... Chances are, if you call up and explain your situation you could get an appointment for the same day. Save yourself the germ-fest and waiting that is the ER.