What Could Cause Irregular Heartbeat?

I have been referred to a cardiologist because I have an irregular heartbeat which is causing shortness of breath and is uncomfortable. My ECG concluded normal LV and RV function with a mile left atrial enlargement (42 mm). In the past 15 months, I've lost 25 lbs, ran a half marathon in December, exercising (with a trainer) 4 days a week, and eating healthier than I have since college (I am 37 years old). My grandmother (deceased) had and my mother has an irregular heartbeat so I feel it was inherited. I will see the cardiologist tomorrow but wonder how I've not truly been affected with the signs and symptoms listed on most websites and in most books. I enjoy exercising and beginning to Crossfit train (heavy resistance training) while continuing to jog/run (to include half marathons). My worry is that I would need to decrease training. I read LAE is treatable with medication. Is it possible for me to continue training/exercising whether or not I am placed on medication? I'm just beginning to feel really good with my exercise regimen and still need to lose 15 lbs. This is a huge concern as again, I would think I am healthier now than I've been in a while. Thank you for your response.