Oh boy, what a story. Good thing you noticed you'd taken the wrong pills!
I know it is a powerful drug, neither my cardiologist nor the consulting EP would allow me to try to drug for treatment of my permanent AFib unless I was willing to check into the hospital for the introduction of the drug.
I have seen many post on this Community of people who started Flecainide without behing hospitalized for the initiation. I think prescribing that drug "over the counter" is common place in Canada (not sure where I read that) and maybe in many other countries.
Not sure where I'm trying to take you post, but it is good to read you are ok, maybe thanks to quick thinking. I think the best practice is to dispose of all "old" drugs we are no longer taking, even if they did cost $$$.