I've never been motion sick, and I've been on everything from theme park rides, small sail boats on rough seas, flying and wild chopper flights in tropical thunderstorms (don't ask) where even the pilot got out and threw up once we finally hit the deck (in one piece I'm extremely glad to say) and I've never had any problems with feeling ill during or after a computer game.
So my first guess is it's motion sickness from the bob movement, second guess is it's caused by eye strain from a CRT with a low frame rate and third guess is epilepsy, you don't have to be lying on the ground twitching for it to be epilepsy, disorientation is also a symptom (they do put those epilepsy warnings on games for a reason)