Hi and thanks for the query,
Thus sensation most of the time is what is called a syncope
or a black out. It usually arises because of a temporary decrease in blood flow and supply to the brain. The causes might be cardiovascular or neurological. But most of the time, when it happens only once, there is no need to worry. At times, it transitory and does not actually indicate presence of any disease.
If it becomes so frequent, then you might need to consult first a cardiologist.
For now, I think you need not worry.
Thanks and best regards,