I dont mean to point out the obvious but I think dizzyness is the least of your worries after spending 18 hours playing a game, Im suprised you were not dehydrated and verging on epilepsy.
Im sure most people would agree with me and see clearly that the dizzy spell was short term and brought on by the excessive gaming.
I find 8 hours working on a laptop as a developer taxing enough on the eyes and circulatory system.
As for getting up suddenly Id say this is not uncommon either. Like the doc said... just an ear infection or the rush of blood out of your head as you got up suddenly from a relaxed sitting position.
my advice... move slowly and refrain from 18 hour stints in darkened rooms... that kind of thing is reserved for gimps
