A couple of things here...number one is that you definately have age on your side and it would be pretty unusual but not impossible to suddenly pop up with heart issues if you weren't born with them. This could be absolutely anything from doing repetitive sports, to things being switched around in your gym class at school, to being on the computer for too many hours...the best suggestion that i would have is to take these worries to your parents...that is what they are there for is to see you thru the scary times of your life so that you don't have to go thru this alone...if thats not possible i would definately talk to the school nurse or your gym coach because they will know what to do and where to lead you. My first train of thinking my friend is no worries on this but yep you need to know the answers if it is scaring you...however the odds are most definately on your side that its not heart related but stranger things have happened...talk to your parents or another person whom is an adult and a good support network for you. Good luck but i would relax a little.
Not to trivialize your question, but I think you have no choice but to ask your doctor about this. Most likely it is not a serious issue, but if you want a dependable answer, a doctor will have to assess this problem for you.