I experience these too. I wore a Holter and was told pretty much what you were.
It's apparently very common for people with panic attacks to have healthy but irregularly-beating hearts. The irregular heartbeat triggers panic attacks when we're younger, and when we get older we start to notice the heartbeats themselves, rather than just the adrenaline flooding our systems as a result.
It sounds like you've been checked out pretty well and have a clean bill of health, which is good! The next step in my life was to improve my diet and start getting regular exercise, which has helped me tremendously. And remember, they checked your heart and it's fine. When you get these fluttering heartbeats, remind yourself that it's just panic, and you can work your way through it.