It's a form of motion sickness.
Minecraft is a first person game, so the screen moves around as though you were moving through that world. Your brain gets confused seeing motion, but not actually "feeling" motion from your inner ears. This can cause dizziness, nausea, etc.
The Sims is not a first person game. So your brain knows you aren't actually moving around - just watching people (Sims) walk around in their house. So there's no confusion, no dizziness, and no nausea.
You can try to minimize the effects while playing first person games by doing the following:
* Play on a smaller screen, or sit further away from the screen. If the game isn't taking up your whole field of vision you won't have these confusing feelings.
* Play in a brightly lit room. This also helps convince your brain that you aren't "in" the game. Just being an observer.
* Play in short bursts and take frequent breaks - even if it's just to look at something else besides the TV/monitor. Again, this is to help your brain understand it's not "in" the game. Also gives your eyes a bit of exercise (focusing on different things near and far) and whatnot.