This:
If you read about what causes a vestibular episode, I think it's quite possible, likely, this is what happened to Jennie. She was just lucky enough (or healthy enough) that everything righted itself very quickly. Maybe even me turning her over onto her back (holding her like a baby) helped things slip back into equilibrium, you know?
I've had Benign Positional Vertigo twice in my life. Long story short, there are calcium crystals called otoconia in the inner ear that are attached to little hairlike projections. These crystals add the weight to those "hairs" and pull them down so the body knows where it is in space (up, down, left, right, forward, back, etc.). Sometimes the crystals detach and float around in the fluid, and if they get out of the area where they're supposed to be, they can cause vertigo, including the nystagmus. The treatment for it, at least in my case, is something called the Epley maneuver, where you turn the head in certain ways to get the crystal back where it's supposed to be so it no longer disrupts the ear fluid.
From what you described, and esp. now that you mention turning Jennie over, makes me think that might be what happened to Jennie. I just did a search and it looks like cats also have those crystals (my guess would be that all mammals do):
http://www.ncbi.nlm.nih.gov/books/NBK10792/ Scanning electron micrograph of calcium carbonate crystals (otoconia) in the utricular macula of the cat. Each crystal is about 50 mm long.
So it's very possible that she had a crystal dislodge (perhaps from shaking her head or tilting her head to scratch behind her ear, who knows), and when you turned her over, you moved the crystal back where it wouldn't disrupt her balance.
I sincerely hope, for both your sakes, if that's the case (or even if it's not) that it never happens again.