I get arguments here, but the literature is very clear: atrial fibrillation (AF in the literature) is a progressive electrical disorder. Except for the exceedingly rare 'lone AF' that some talk about, especially after a virus or a bout of heavy weekend drinking, once the heart begins to develop an arrythmia, it begins to remodel itself. In fact, truth told, the remodeling had been going on for some time, and at some point it causes AF.
Your Kardia is a good machine, but it's not a substitute for a 12 lead ECG. With the reading of 240 BPM, it was highly unlikely to have been AF. Instead, it was probably flutter. AF runs, believe it or not, between 80 and 160...typically...and yes, one can be in AF with a low HR of only 80. It's not what I would call common, but it does happen.
Flutter and AF both do respond to cardioversion if done early, say in the first 12 hours or so. Trouble is, flutter tends to like its own company...it will stay in flutter if given any help whatsoever...including riding it out and hoping for the best (meaning avoiding seeking help). So, treat both of them forcefully and early.
I have AF, but as I have related here and there in replies to people, one time I was in the ER hoping for relief, the attending wanted me given adenosine to slow my heart so that the ECG monitor would show a longer, wider readout...meaning more definition in all the squiggles. When I was at my worst with the adenosine (nurse warned me it makes you feel like you're about to die, but that it only lasts a few short seconds...and she was right, if anyone is asked to submit to that trial) the attending triumphantly pointed to the screen and said, 'There it is...flutter.' So, what do I know...it was flutter, but my formal diagnosis has always been paroxysmal AF. Anyway, they converted my heart to normal sinus rhythm (NSR), but it only lasted for about 16 hours and I went back into AF, back to the ER, another cardioversion, and back into AF soon after I got home...and on and on it went until I had my second ablation.
You are having more AF. Sorry, but it is so common that an ethical EP will tell you up front that his/her first attempt at ablation has about a 25% failure rate. Happened to me. Second attempt is generally better, about 85% success rate, and I was given the nod by the Fates on my second go.
If you're not already doing so, a smart watch or a smart ring (Oura or Samsung), will detect AF and will allow you to upload a viewable ECG, as will your Kardia. Most physicians who aren't still living in 2012 will accept the finding and act accordingly. If you are symptomatic, accumulate such records when you are in AF, and seek help from an EP.