My Daughter Has Been Sick With A Fluctuating Heartbeat, Dizzy

My daughter has been sick with a fluctuating heartbeat, dizzy spells and feels like she has a knot just below her rib cage she has been in the hospital twice in three weeks and they can,t seem to find the cause she is a rn and went to disneyland a month ago and had all her shots but got ill eight days after returning home. Tests seem to come back negative and she gets tremors and shakes all over when she feels sick. She was taken by paramedics to the hospital twice in three weeks, please help if you can thank you,