So I'm 13 And My Heart Beats Really Fast .

So I'm 13 and my heart beats really fast . I know it's normal for your heart to beat really fast when your anxious or nervous. But I could just be laying down relaxing and I can feel my heart beating really fast without even touching my chest. I've been to a cardiologist before they said I was fine. Do I have like really bad anxiety? Or should I be worried??