Atrial fibrillation is a disorder of heart beat in which the upper part of the heart (right and left atria) beat at a very rapid rate but the pumping chambers (ventricles) are regulated at a lower but irregular rate usually more than 100 but can be much faster upto 180 to 200 unless treated with drugs.
Your heart rate being much lower seems to be normal and the atrial fibrillation reported could be artifactual, being a computer generated report. If atrial fibrillation is confirmed an echocardiography needs to be done to look for any underlying structural cardiac problem.
I think you need to be evaluated in view of unexplained fatigue etc.