Ok, so sometimes my heart beats really fast and i have trouble breathing. My chest tightens and it hurts. This usually lasts for about fifteen minutes. When this happens, i zone out and i want to be alone and away from everyone. This has only happened like two or three times, and I'm wondering if this is anxiety. I don't really get triggered by anything, nor does this happen when i get scared. most times this happens after running a short distance or doing some sort of physical exercise. I've been to a heart doctor and they found nothing. please help i need answers!!!