I have this all the time... I actually have shoulder issues and once I got my shoulder straightened out (literally!) my chest didn't hurt quite so bad. It is really alarming to have that area be sore though!!! Have you tried taking some ibuprofen and resting your left arm? I know when I paint or groom my horses it flares up, especially since I'm left handed.
Hope you are feeling better!
Oh yeah and if I have an upset stomach it gets sore as well... like weird gas pains. Sorry that was an afterthought.