"It's actually quite sad, as it's a sickness, not "playing"

Your first message literally states as of it were fact that the cat is sick, then you follow it with " might be nothing much" and acknowledge that's it's hard to diagnose with a 2 second gif. It makes it sound like you're pulling random comments out of the air just to hear yourself talk.

Have you met a younger cat? Have you known cats when they "get zoomies"? As strange as it may look, this is pretty damn normal silly cat behavior when they get really excited. Ever see a dog playing that gets so excited it startles itself with its own tail? It's the same thing.