dbac-
Smurfy had me laughing too, only because I have been to the ER twice only to be diagnosed with muscle spasms. Once was in the center of my chest and once in my left shoulder. Embarrassing. I will probably die of a heart attack because I will think I am having another muscle spasm and don't want another embarrassing ER trip! Only kidding....if I really thought I was having a heart attack I would go.
Anyway, I have done some research on rib pain, as I have it a lot. It varies in location and intensity. I don't have to be doing anything or have done anything that seems to cause it.
Costochondritis is a possibility. It is not deadly, just a pain in the ribs. Usually only one, but what it boils down to is the cartilage that attaches the ribs to the breastbone can get twisted or otherwise messed up (sorry not a good medical term but you can look it up 
). I have had this happen where a rib ends up twisted and pressing on my lungs so it hurts to breathe. One doc misdiagnosed that as a trigger point when it was actually a rib out of place. If only one doc had ALL of the knowledge! Now that I know what it is I don't panic anymore, my PT has put more than one rib back into place.
Another possibility really is just ("just?") muscle spasms. There are so many muscles attached to our ribs. I have myofascial pain dysfunction and in the past my PT has found trigger points where the muscles attach to the ribs. They are deep so very hard to treat, if at all.
Sometimes what helps me is to locate the part of the muscle on the rib that hurts the most and apply gentle pressure for 15-20 seconds. Not massaging, just gentle pressure. It will hurt. Again, sometimes it helps, sometimes, not.
I chalk it up to another fibro symptom, but if you are concerned see your regular doc. ER docs sometimes miss the simple things since they are looking for major trauma.
Good luck!