The symtom may well be due to a sensitivity.  I would recommend contacting your nearest member of the British Institute for Allergy and Environmental Therapy, who will be able to determine both the likely cause and treat it.

I also had this condition for many years.  It is often affected by weather conditions, and I would strongly recommend your grand-daughter is encouraged to be calm if there are storms around.    In a child, after a short period of fast heart rate, there is a tendency for panic to set in, which serves to increase the heart rate ...