Welcome to my world. Do you drink a lot of caffeine. Let me tell you, as I have had it for years. First you feel the fluttering and then its frightening so you have an anxiety attack which increases your heart beat, and pretty soon your in a full scale panic attack.
What I have learned to do, is close my eyes, and take slow cleaning breaths, exhaling as much as you can and inhaling slowly and shallow ie.
They also gave me beta blockers but I found that it just made it worse, so I spoke with the doctor about going off of them and I did. If your doctor did all the tests, and found nothing, then your find and it was to much caffeine that da.