She may have rapid heart beat without an underlying heart disease.
This can be a symptom of overactive thyroid gland, anemia, excessive use of tobacco and coffee, she may be easily excitable, suffer anxiety, have chronic lung disease, or this can be a side effect of certain drugs like asthma and allergy medications.
An occasional rapid heart rate, can be a Paroxysmal supraventricular tachycardia, that appears especially in young people.
Best wishes!
Marija