It could be nausea due to a low field of view in the game messing with your head, causing you to feel sick. Your head is used to seeing a larger field of view, so if something that only provides 60 degrees of vision takes up more than 90 or 100 degrees of your actual vision, your brain doesn't understand it and causes you to feel sick.
Either try sitting farther back from the monitor, or turn up in-game field of view and see if that stops it.
Or you hate the game completely, so you feel sick when you play it.
I'm pretty sure it's the former though.