What kinds of heart issues can a healthy, active child develop?

I'm trying to write a short film for my film studies class, and am having a hard time finding anything having to do with heart diseases in healthy children developing or worsening.

The idea of the story is that she is no longer allowed to go to rigorous dance classes due to her heart problem- but she once was top in her class, meaning that her heart problem didn't previously prevent her from hard exercise.

So far I'm thinking it could be a heart murmur that was incorrectly thought to be innocent, but which caused her problems as she got older. But I'm not entirely sure.