Chest and back and left shoulder pain, slightly numb left arm for weeks (like having cramps), hard to breathe. Gerd? Or more than that?

More than that. Gerd does not typically cause any extremity symptoms. The pain and numbness and difficulty breathing are worrisome, but less so given your age and listed lack of any other health issues. Much of your pain could be caused by a cervical spine issue, but not shortness of breath. Get in to see your pcp.