Sounds like it could be physical anxiety. I suffer from physical anxiety even when I was smoking. Its hard to process as most people resemble anxiety as a mental feeling not a physical one, and when you can feel your heart beating but in your head you feel like you're fine it can be scary. Once i visited as doctor and they made me aware of this its really helped, as every time my heart starts palpitating/fluttering I know its my anxiety and I calm myself down.

I'm not saying this is what it is, but sounds very similar to my anxiety. If you've only experienced this after quitting, and not before, I very much imagine that it'll pass as your brain balances out :)