Heart beat suddenly increases.

Hi I am 46 years old, very fit, don't have any health complications, i get master health check up every six months and all the reports come absolutely normal. I walk 6kms everyday and i am fitness freak. I had ECG checked up, its normal. I am not on any medication. Last year TMT came as positive and immediately i underwent coronary angiogram and its absolutely normal. My issue is whenever I read or hear any heart attack news who is younger than me, suddenly I feel like I might be victim of that in coming days. My heart beats rapidly and mostly this happens in the middle of the night..I try to calm down considering my fitness and reports which are normal. This feeling disturbs me atleast couple of days from the day i listen these type of news..kindly suggest any resolution which can be helpful and to come out of this disturbing situation.. Thanks in advance..