Hi moon sun and welcome to the cardio forum!!
Other diseases can mimic heart disease, but I would never take a chance on that being the case!! EKG's are a fine tool, but cannot always tell if something is happening.
You may need to have other testing done. I would definitely recommend you at least discussing this with your Dr.
Now with all that said, I want to explain that I am not a Dr. and most of my knowledge comes from what I've had either happen personally to me, or from learning through others.
Fibromyalgia as I'm sure you are quite familiar with, can also bring on some chest pain symptoms. Have you ever had acid reflux problems like GERD, or hiatal hernia? GERD has been known to bring on some of these very same symptoms as well-including the arm pain!-I can vouch for that, as I have had it happen.
With this knowledge in hand I would ask for a stress test at the very least and perhaps an echocardiogram. Both very non-invasive tests and perhaps would show up any problems. Also, ask your Dr. about a barium swallow to follow up with the acid reflux or GERD theory. Again non-invasive, you just swallow something and they then X-ray it going down.
Best wishes to you and I hope this info helped you out a little. Keep in touch, please and let me know what happens!