Is this something I should worry about?

Since I was like 13 my heart sometimes starts to beat very rapidly (in beginning I can't even feel individual heart beats) and I get this nasty feeling in neck that makes me want to cough at first and then there's just sort of thighness around throat and in upper side of chest.

Usually passes when I sit or lay down. Except this one time in my teens it started when we were playing dodgeball at school in hot sun, I tried standing still, but it didn't help and so much blood rushed to my eyes that I could barely see, luckily teacher came up to me and led me to shade.

After that I went to my family doctor, who just suggested I avoid physical activity. Around that time I read somewhere that growth spurts can cause such problems, because cardiovascular system can't keep up with the rest of the body. Plus it hasn't ever been a regular thing - happens like twice a year. So fair enough reason not to worry. Except since then I've had this happen even when I haven't had any exercise and for no obvious reason.

Basically the thing is that I am almost 25, I figure that if it was just growth thing it should have sorted itself out by now, but it happened again few months ago, so I've been doing research online at times when I have nothing better to do. Can't find anything like this. My main concern is that since I don't know what causes it I am somewhat scared to exercise.