I'm Trying To Figure Out What Could Be Causing My

I'm trying to figure out what could be causing my daughter's symptoms. She's already been to the ER a couple nights ago and waiting on a follow -up. So, not life-threatening. But she is 16. She's getting rapid pulses - the other day it was over 120 for 12 hours or more. Cauzing dizziness. Extreme body tremors. Feeling faint. Nausea and some abdomen pain. She can't relax. It happened again this evening, but her pulse and trembling seemed to calm down after a shorter period of time. They ruled out infection. Also took heart xrays which looked clean and an EKG which was very slightly abnormal, but the pediatric cardiologist said that unless our PCP sees the need, he won't worry about getting her in. We see him on Monday. In the meantime, we're looking for ways to keep her pulse down. Any ideas on what could be causing it? She was so wiped out after the ER that she's just laid around, but tonight it happened again. It is really scary for her.