Myeh, there are a lot of other possibilities - I've given blood several times and they check your blood to check if your aneamic etc - so mine at least certainly aren't down to that.
I'd put it down do being tired, dehydrated, low bloodpressure perhaps.
Of course, this does seem to be the sort of thing you ask your parents about, and if they don't know and you're concerned, ask your GP next time. It's hardly something any of us can diagnose, and any med students (the kind likely to be on here) around are bound by law not to give medical advice.
So yeah, maybe ask a GP, but sounds like you just need to rest