vasovagal syncope
I used to get that all the time back when I was really pushing myself hard training for long distance running. I never fully lost consciousness. It happened when I would stand up quickly. The dizziness would go away once I dropped to my knees. Sometimes my vision would go dim and my hearing would fade for a few seconds. I think it was because intensive long distance running drops your resting heart rate and your resting blood pressure to ridiculous low levels. My resting heart rate was 40 beats per minute and my resting blood pressure was about 110/48.
MY resting BP is about 130/89.................but I’m 57...........