You may very well have had AFib as confirmed by a cardiologist, so don't take this comment as attempting to say you did not have AFib. It's important for those reading this to note that flutter does not equal AFib, at least not in many (most?) cases. Afib is characterized by the absence of regularly occurring P-waves, and yes, one will often feel fluttering.
The fluttering I experienced was caused by a heart beat beginning too soon, causing a noticeable pause between beats. (EDIT/ADD: The premature beat also feels like it came too quickly. One will sometimes feel their breath catch when this happens.) It's probably the most common type of flutter. At worst, this occurred many times per minute, but never seemed to affect me when trying to improve my fitness.
I had fluttering for more than 30 years before being diagnosed with chronic AFib. It (the fluttering) had approximately zero affect on my ability to do hard work, or exercise vigorously for hours. Chronic AFib on the other hand, has been debilitating. Chronic AFib refers to continuous AFib present for more than two years, that is, one's heart is never in normal sinus rhythm. Average life expectancy for someone like me, who was diagnosed about mid-sixties, is about 10-years. The loss of energy and endurance is significant.
Paroxysmal (intermittent/episodic) AFib is more common and can often(?) be cured by ablation and medication. Ablate and pace (pacemaker) is another approach.
Ablation for chronic AFib is sometimes attempted in cases where open-heart surgery is sometimes required for another purpose, as ablation is required both inside and outside of the heart and pulmonary vessels to be effective for chronic AFib. Ablation via catheterization is rarely effective in chronic AFib. However, a company named Atricure is experimenting with a catheter-like approach that utilizes three(?) catheters to ablate both inside and outside the heart and pulmonary vessels. They claim improvement in about 67-percent of their attempts, if memory serves. It's an understatement to say it's a rather involved procedure.
For those with chronic AFib, the treatment is often just anticoagulants and rate-control meds, e.g. metoprolol.
(EDIT/ADD: For those with paroxysmal AFib, treatment often consists of rate control, anti-coagulants, and anti-arrhythmic meds. Sometimes cardio-version is required to restore normal sinus rhythm.)
Anyway, I'm not an expert, just a layman with a dog in this fight. Hope a cardiologist chimes in.