--I have a recurrent left lower shoulder blade/rib/trapezius/latissimus problem that like to act up from time to time, especially when I overdo weightlifting, and often the pain/discomfort comes around the rib curve towards the front, in the general area of where the lowest ribs connect to the sternum, but not quite all the way to the sternum.

From some of the physiatric literature I've read it's not uncommon for rib connected muscle pain to reflect to the side of the chest or even the side of the stomach.