What does fast heart beat and shortness of breath mean?

It can be from anxiety, over exertion, Heart disease, and high blood pressure, among other causes. If you are having any chest pain, pain down the left arm, or a tightness or squeezing sensation in your chest, or nausea, call you doctor immediately. This could be signs of a heart attack. If you are not having any other symptoms, but the shortness of breath and rapid heart beat persists, you should call your doctor just to be on the safe side. It's always better to err on the side of caution.