Hi; I'm Emma Claire; 16 years old, and for the past 6 months or more have been dizzy, lightheaded and my vision goes black when I stand up. It goes away in a few seconds, but when it goes away, my heart starts beating really fast. I have been tracking it for the past two weeks now and it occurs at least twice a day; with the max being 5 times a day. I am 5 foot 7 and 110 pounds and do competitive cheer so I'm always active. I have also never had my period if that makes any difference/or is of any help. Does anyone know why this could be happening? Thank you so much in advance because I'm kind of worried!