As has been said, this realy needs diagnosis by doctor not diagnosis by internet
BUT
Young adults can have vasovagals where the heart slows down. In my youth it was 'fashionable' to deliberately cause loss of consciousness by a valsalva manouvre where you grunt against a closed mouth - same as strainingf if you are constipated.
you can also feel faint if you suddently stand up from a hot bath due to low blood pressure
And you may have a low blood sugar after binge drinking.
Apart from these 'normal' events all loss of consciousness is abnormal be it from arrythmias, fits, or low blood sugar (rare tumours unless you are diabetic and having a hypo but then you wouldnt be on pprune!)
So sadly it isnt normal in most cases, needs investigating, and may recur so you cant fly or drive. However, in practice we normally find out the cause of determine it is 'normal'