It would still be an arrhythmia, but it could be attributed to several causes, not least of which might have been in your future simply due to aging and genetics. You go from tachycardia to bradycardia (fast heart rate to a slow heart rate) with no obvious stimulation, or at least one which you know about and have shared.

So, you may have SVT (supraventricular tachycardia) that swaps places with bradycardia due to sick sinus syndrome ( https://www.mayoclinic.org/diseases-conditions/sick-sinus-syndrome/symptoms-causes/syc-20377554 ). Again, I'm just fishing here. I have no medical expertise whatsoever. You should consult a cardiologist soon and I expect a pacemaker is in your future, but certainly a visit to an electrophysiologist for a formal assessment.