The first thing that has to be ruled out is an arrhytmia, or abnormal rhythm of the heart induced by exercise, which are possible. The best initial approach to accomplish that would be to arrange for a Holter monitor (little pager sized device that is connected to your body) so that if symptoms arise with physical activity, the machine records the electrical activity of your heart, and you can actually record in a diary any symptoms that you have. Then, information will be interpreted by a cardiologist. In many cases, there is no heart abnormality, just the normal rhythm, called sinus rhythm, going faster (as expected) with exercise, and some people (particularly thin females) are more aware of it Hope this helps, Dr Brenes-Salazar Mayo Clinic Rochester