This is actually VERY common, it's called simulation sickness(kinetosis). It happens most of the time during first person games or anything that uses fast motion. Apparently they ran some test in a school where 100 people played Halo for an hour non stop and around half of them felt sick afterwards.
What happens is your eyes tell your brain you are moving whereas your inner-ear tells your brain you aren't moving. This causes a conflict within the brain and it thinks it has been poisoned so it induces sickness as a way to try and rid your system of the poison.
In your case your eyes are telling your brain where an item on the screen should be, however with the framerate issue the item isn't where your brain thinks it should be, and with all this happening at quite a fast pace it can confuse your brain and again, induce sickness to try and rid itself of any poison.
I find the best ways to stop it happening are playing in a very cool environment with a fan blowing at your face lightly, also try and sit as far away from the screen as possible and if you do start to feel a little hot and sweaty, stop playing immediately, lie down flat on your back and close your eyes. This stops the conflict with the eye and the inner-ear as your eyes are telling your brain you are asleep and your inner-ear detects no movement. The brain thinks the poison has been flushed and corrects itself.
Can you tell I researched this? It's how I cured my inability to play FPS's. I love them now whereas about a year ago I wouldn't be able to play one for more than 10 minutes.