What Causes Rapid Heart Rate Along With Pain In Abdomen Of A Child?

hi yes i have a healthy 3 year old son and starting this morning he is saying he can feel his heart beat and he is holding his left abdomen area. He knows where his heart is located and everything so now he is scared and just started crying because he doesnt think his heart should beat by his left abdomen. I think he may just be having a muscle spasm or could be getting himself worked up and causing himself to panic. What should I do to explain that this is nothing to worry about? or should I just take him to the doctor to be on the safe side?