‘VR motion sickness’ is when physical discomfort (for e.g nausea, disorientation) is experienced by a user when in a virtual environment.

There are a few theories why this occurs, ranging from technical to human physiology. For example when conflicting signals are sent to a person’s brain from their eyes, inner ears and sensory signals telling them they are moving when they are in fact stationary. 

VR sickness is an area still undergoing research.

There are steps developers of VR environments can take  to reduce any discomfort for example reducing non-forward movements and reducing user movement in the virtual environment.