Sometimes my heart like feels very strange, it sometimes like misses a few beats what is it?

Im relatively healthy, and this holiday i have done loads of exercise and i dont really eat lots of junkfood. But i can just be sat there and my heart will like miss a few beats, it doesnt hurt. But im only 16 and i worry if it may be like a heart attack or if im going to have heart problems in the future, what do you think it is?