As you know not a cardiologist but when you mention very strenuous on you that might have been the trigger?

Maybe talked to or send a message if you have a portal to your cardilogist or if you have one your EP. Not knowing your health history and even if did not a medical professional over doing it is not good for cardiovascular system if you have medical issues especially cardiovascular.

I am allowed to do Sprint Triathlons even though I have heart failure and have a ICD/Pacemaker. But I had to promise would do a moderate level and not maxium effort at any time. So I am slow and back of pack but really enjoy doing them. When people see my age on back of my leg I hear "oh what an inspiration, I hope I am out here at your age still racing."