I've had similar experiences. One that stands out the most was from high school. I had never met my friends hamster and as far as we all knew, it was healthy and okay. I jokingly told her "oh yea? ...

I'm still trying to figure out what exactly this is but you're not alone. I have a beta fish in my office at work and normally it just nose dives to the bottom or just sits still for hours at a tim...