Hello Atrial fibrillation is closely linked with age. The older we get the higher our risk is of developing it. It is extremely rare for atrial fibrillation to develop in younger patients, unless they have an underlying heart condition. Experts say there are many cases in which the cause of the atrial fibrillation was never found. The general causes fir atrial fibrillation are Hypertension - high blood pressure. Coronary artery disease Congenital heart disease -  Cardiomyopathy - a serious disease in which the heart muscle becomes inflamed and doesn t work as well as it should. Pericarditis - inflammation of the pericardium - a protective lining that surrounds the heart. Previous heart surgery - Hyperthyroidism - the thyroid gland is overactive. Sleep apnea Alcohol  Smoking  Excessive caffeine consumption - this could include too many coffees, energy drinks, and or sodas (fizzy drinks). Many sodas have caffeine in them Several types of chest infections and conditions can also cause atrial fibrillation like Pneumonia Lung cancer Emphysema Chest infections Pulmonary embolism Carbon monoxide poisoning It can also affect very athletic persons and probably this was the cause in you since no above mentioned causes could be found out in you. Thanks