What Causes Pain In Chest And Rapid Heart Rate In A Child?

I have a six year old little boy who is a first grader and he was fine this morning and all of a sudden he told me that his heart started hurting and he said that his heart was beating very fast...He had to get out of his car seat to sit on the regular seat to feel better.....this was on the way to school this morning.....I felt his chest and I could feel his heart beating very quickly...After about 8 minutes he said he felt a little better and he could go on to school...I called the teacher 20 minutes later and she told me she would watch him but that he was being unusually quiet...what do you think?