Please seek another Doctor's opinion.
It could be a lot of things so tests should be made.
When I was your age, I would collapse as well, black dots in my vision, collapse hit the ground, blacked out but re-awoke, it was diagnosed then as low blood pressure.
It went away eventually, many, many years ago.
So it could be deficiencies in your diet as well.
Get a blood test and any other test you need. This would be frightening to you and don't ever listen to people that google and try to give you answers, you can only get suggestions via the internet and ultimately, Doctors to confirm.