It is something that many folks are experiencing. Myself included. My cardiologist rules out any heart problems, was not A fib. GP put me on anti-anxiety meds, did not stop the fluttering. I found references to 'internal tremors' so that is what I am calling it now. The Mayo Clinic has a video on Central Nervous System Syndrome that might be helpful to you. Read back through the comments and you find a link. Good luck.