I have the same thing. My sore spot is right along the upper, inside left breast. It aches strongly at times that it's almost nauseating. The pain will radiate into my shoulder, arm, and neck. It's a very scary feeling.
My doctor told me it's probably costochondritis. Since I can push on that tender spot and make the pain worse, it's not heart related.
The feeling of costochondritis mimics a heart attack so it is scary and uncomfortable.
Mine comes and goes with stress. Right now it's back due to a lot of stress. The past two nights I've been taking Slave and using a heating pad. Once I start to relax, it let's up.
If you are experiencing the same thing, check with your doctor but it certainly sounds like what I have. Hang in there and try to relax.