As mentioned above, there are a lot of possible reasons.
I've had a lot of experience with the motion sickness due to response time. I can probably help you find out if this is an issue for you or not.
If you can load your most demanding game, and put it at settings that give you about 30 average FPS, and play for 30-60 mins. Try to figure out how long you had to play until you first started to feel eye strain, then rest up or even wait another day so the symptoms are gone. Now lower the settings as low as they go, or at least low enough to get 60+ average FPS. Now play a while. If you can play a longer period of time before your eye strain kicks in, you are like me (though I feel nausea, but both are motion sickness symptoms).
If the higher FPS made it take longer before the eye strain kicked in, it is likely response time causing your eye strain. Insuring you have 60+ FPS will help you feel better, getting a 120hz monitor and getting over 80+ FPS may even eliminate the eye strain. That is what it takes for me.
As mentioned above, it could be a number of things, this is just one I have experience with personally.
EDIT: The motion sickness problem will only occur when controlling the action of a game. It will not occur when watching movies. It happens most often with 1st person games, but over the shoulder games also give me the same issue. Playing a game where the view is controlled by your mouse gives the most motion sickness issues. When testing, try to pick a first person game, like a shooter, that uses the mouse for aiming.