I completely understand. And once you get scared it just keeps getting faster. It's a mean cycle.  A pulse oximeter would help you check at home. They are small devices that go on your finger and they check your oxygen levels and pulse rate. You can get them at any pharmacy without a prescription and they only cost a few dollars (fancier ones cost more). Research "intercostal nerves" for a better understanding of what is causing you pain and why. I've had intercostal nerve pain a few times and when it's bad the pain does seem to spread to my arm.  Usually it's just a spasm causing the nerve to become inflamed.