Hi,
This has happened to me 4 or 5 times... you may have "vasovagal syncope" (situational fainting) and it's likely related to the low oxygen on the airplane... I usually get it after I've fallen asleep for a few minutes, on the way up (I've not gotten it on descent)-- you breathe more shallow when you sleep, so get even less oxygen (and it's low on airplanes to begin with).
I wake up feeling sick, get up, and pass out near the bathroom (never usually actually sick).