A 14yr Old Girl Extremley Fit (swimmer) Passing Out On

A 14yr old girl extremley fit (swimmer) passing out on ongoing basis but not loosing conciousness. Episodes followed by chronic fatigue. Lately some numbness in legs and hands during collapse. A and E few times but no answers other than irregular heartbeat in ambulance and trace in A n E last time. What can it be??? Desperate for answers and waiting for appointment.