My daughter is grown, but when she was about 12, she complained of the same symptoms. Had to wear a heart monitor for a MONTH...nothing showed up.
I think she was just having anxiety, as she later developed pretty bad panic disorder.
Is anything going on at school? New teacher? Friend moved away? Kids teasing?
I would follow up with the pediatrician...don't take no for an answer...ask for every test that seems reasonable. Good luck...its scary when our kids don't feel well, but I would try not to worry. God Bless...
It could be a number of things, she needs to be seen by a Doctor asap.