Simulation sickness [http://en.wikipedia.org/wiki/Motion_sickness#Simulation_sickness] is a not-uncommon issue amongst gamers (and would-be gamers). In the case of the Portal series, I've found that the combination of rapid movement, orientation changes and motion blur (as some people have this enabled, and I think Portal 2 does it by default) really messes with the equilbrium of some players.
I like to suggest the following possible remedies (trying one at a time): Play in windowed mode, switch between first/third person view (if possible), lower the resolution, sit further back from/closer to the screen, place a small object in view of the screen, make sure that extra effects (like motion blur) are turned off.