My 7.5 year old, healthy son, woke up at 5am crying that his heart is beating fast and hard, and that it hurts. We lay him down, got him to breath deep breaths and relax, but his heart continues to beat out of his chest. He was complaining that when he's standing, it hurt more. After 15 minutes, the pain went to his stomach, but his heart is still beathing hard and fast. What could this be?