Where does it come from? Why do my senses seem to play this trick, this prank, on me sometimes?

I sit here, noding away, when I realize there seems to be a sensation of descending throughout my body. If I remain focused on the monitor, I can just know that I'm somehow moving downward, my entire body. Then, I look around, and the feeling disappears.

A few moments later, back on the monitor, I can feel it again. Right now, it seems to be going sideways. I feel like I'm tilting, leaning, and I almost want to fall over to one side. Finally, I can't take it, and have to move, to look elsewhere, make it go away.

But every time I go back to the monitor, it comes back... oh, go away.... please...