What type of heart conditions only show up during exsercise but not at any other time?

Yes but often early: There are many heart conditions which will show up only during exercise or exertion. Often this is an early sign. Our bodies have a lot of reserve. So when we are at rest or at only moderate levels of activity there are no symptoms. As the heart get sicker the signs show up earlier. Some examples include blocked arteries, blocked or leaking valves, a hole in the heart (often congenital).