Ectopic heartbeats during exercise

[Spytum]

Hi
I have general anxiety and continually worried about my heart. I exercise everyday ( about 40 minutes of which is cardio ), am 18 and otherwise healthy.
Occasionally, ( has occured around 4 times in the past 8 months ) I feel an ectopic beat, which varies in character from one massive heart beat to just a sink in my throat. This occurs whilst I'm running or doing other relatively high intensity exercise. It lasts about a second and then goes and I feel completely normal but very worried. I have had another recently and am worried to fully push myself again incase I experience another.
Has anyone else experienced this during peak exercise? I've heard it's common in the cool down phase but this is definitely just during high intensity or perhaps if I am anxious during the session.
Thanks

[LeMa79]

I get this, always when I am breathing in. However I also get them sometimes when I yawn or take a deep breath so presume it is a vagus nerve thing or something. I have had a holter and an echo and nothing sinister showed up. I exercise 4-5 times a week (cardio) and am 34 yrs old. I still freak a bit, but am trying not to let this rule my life.