It might be possums. I had an urban possum encounter and I got confused. They look like a cross between other animals and a first encounter makes you do some thinking.
While viewing the necker cube and “willing” it to switch orientation back and forth I did get a slight, momentary feeling similar to nausea. It was odd.
Could it be fear related? Maybe the visual confusion is triggering a fight or flight kind of reaction, only there’s nothing to fight or flee? I know I get stomach problems when stressed or scared. Although I also have unrelated stomach problems even when not stressed or scared so maybe my experience isn’t even a good anecdatum.
ETA, I too have had the experience of mis-identifying something–“Holy shit that’s a huge cockroach oh wait it’s just a piece of bark.” But it’s not accompanied by nausea as far as I remember.
Yeah, I’ve had that one, too. Twice. Once with a “cat” and once with a “squirrel”. Fortunately, both times, both I and the other mammal minded our own business and kept our distance.
EDIT: And while I wouldn’t describe the sensation as “nausea”, it is a bit like dizziness. Sort of all of the parts of dizziness other than nausea.
Similarly, I saw a big male wild turkey and my brain screamed “cassowary!” True, I had recently moved from Australia to California…
I’m reminded of the story of the “red spade” experiment (asking people to identify playing cards and slipping a few weird ones into the mix): https://bornintocolour.wordpress.com/basics-of-culture/perception/playing-cards/
Sometimes this happens:
“And the subjects who then failed often experienced acute personal distress. One of them exclaimed: “I can’t make the suit out, whatever it is. It didn’t even look like a card that time. I don’t know what color it is now or whether it’s a spade or a heart. I’m not even sure now what a spade looks like. My God!””
That happens to me when im expediting the food line too long at a restaurant.
The words stop making sense, i can’t read them and associate them with the food until after I’ve taken a break.
I think what the OP is talking about may be related to something called “visual vertigo” or “visual vestibular mismatch”, “an inappropriate response to motion of the visual environment due to overreliance or misinterpretation of visual cues”. This is also called “visual dependence” or “see sick [not “sea sick”] syndrome”.
More specifically, there are optical illusions called “anomalous motion illusions” that are well-known for making some subjects feel dizzy or nauseous. Here’s a description by a computer scientist of some of the technical issues with anomalous motion illusions, and here’s a classification of different kinds of AMI’s.
So as other posters have suggested, I’d guess that the sudden perceptual shift from one animal to another is acting on the OP like one of those optical illusions. Why exactly optical illusions can make people feel dizzy or nauseous I don’t know.