now you know why i'm not a supporter of the fps genre in games. the only fps game i've ever played and manage to beat was metroid prime 1. do you know how long it took me to beat that game. there are other games that caused these type of feeling from non fps games, like silent hill [camera system was awful] or the suffering.
i believe it has something to do with the point of focus. i noticed that when i play prime 1, i often resorted samus to becoming a ball and walk around the corridors that way until i meet an enemy or something. i believe the problem is that the player can't keep a central focus of the screen and therefore when you move, your eyes aren't focusing on a particular spot on the screen and you end up with the nauseated feeling and the headache.
i though the doom days were bad [they were linear], then i saw people playing unreal and i'm like, these people are crazy. i believe the console players got it easy as compared to the pc in terms of the fps games. if you watch a pc gamer play unreal, the screen is moving so fast you sort of wonder how do they endure such frenzy.
as a side note, if you feel this way when playing fps games or games that provokes such a feeling, it's best to avoid them and don't play them. the result would probably lead to a seizure or something if you force yourself to go further.