I have Afib but it doesn't always cause fast heart rate. My heart rate drops to forty with frequent 4sec pauses, hence I have a pacemaker with a floor 60 at BPM. It depends what other conditions affects the hearts natural pacemaker re sinus node problems.
When I exercise my heart seldom tops 100bpm.
Slow heart rate can imply one is a very fit person, but not in my case.
It is avrelatively simple job get an ECG done at the surgery to see if you have Afib.
It's best to find out because Afib increases the stroke risk by a factor of x5.
The normal approach then is taking an anticoagulant.
D.