I'm pretty sure I get this, too. It happens if I start a new exercise routine or overdo it. I think it's over-stretching the muscles. In fact, I'm sore there right now (kind of in between the breast area & underarm). Both sides are sore, but my left side is more sore. It's annoying, but will go away after a few days. If you feel your left side in the same area is that sore, too? I know for me sometimes one side will be sore without even touching it, while the other feels fine. Then I feel the other side & notice it's tender, too. If this isn't happening for you, then you probably just pulled one side. I'd give it a few days. I'm sure it'll go away.