Presyncope might be caused by orthostatic hypotension. It means when sudden
position changes such as from lying to standing leads to low cardiac output.
Your brain didn't get enough perfusion as needed.
Dehydration from recent illness can be a reason. Even orthostatic
hypotension could be divided into neurologic and cardiac sources. You could
go to your PCP to evaluate situation. Anemia for young females can also be a
cause.
You could alleviate the problem by slowly changing position from lying to
sitting, squatting when symptoms happens, consume some high salt foods if
blood pressure low, head up during sleep, excercise to increase red blood
cells, and on.
Google orthostatic hypotension from position changes, you will find a lot of
stuff.