Could very well be genetic, could have to do with your weight. Otherwise I would say diet, caffiene - though if you don't consume much, probably not), lack of exercise, etc, etc. It's often a combination of genetics and something else though. Your doctor should be a bit more knowledgeable here.
This might help too - http://www.webmd.com/heart-disease/atrial-fibrillation/atrial-fibrillation-overview
::ADDITION:: You may not be obese
but you are technically overweight
which can be a contributing factor of many health issues. Secondly, many minor health issues have little or no explanation. If you read these answers, a lot of people have similar issues and simply deal with it. It's not going to kill you, and I know it sucks, but you just have to deal with it. GENETICS. If you want further explanation the best resources you have are ongoing studies in health and medical journals. Check with a University Hospital for that kinda stuff.