Sim Sickness is real. Even as a Sim Instructor I had a bout of it....pretty easy to cure....find a big thick Oak tree and hug it tightly for a bit.
Usually it is the 47 pints of beer the night before that gets you. Nice thing the law does not apply to Sims as it does for the real thing.
Visual maneuvers seem to be the usual cause....the Sim is moving but not like the visual shows it....your hind end is used to certain sensations when your eyes are telling your brain something else....add in the slight delay between the Visual responding to control movements as compared to the Sim response and you have a good start for Sim Sickness.