I didn't see it myself but from what DH described she had a seizure - keeled over, paws shaking for a few seconds then righted herself, wandered round confused for a few minutes.
She seems more or less back to normal now, she's had a big cuddle and a handful of dreamies which she ate with no problem.
Vets tomorrow for advice, but - she's 22 and way past the age I'd put her through any distressing treatment or even tests unless they might indicate something easily treatable.
Her general health is pretty good but she does get slightly addled at times, e.g. pooing next to the tray then climbing in to try and bury it, occasional nighttime yowling but settles as soon as you pick her up. She still gets lots of enjoyment out of life, loves food, cuddles and sunshine, occasionally still plays wrestling, still sometimes wins!
Is this a sign that we're nearing the end? Is there anything it could be that is simple and non-invasive to treat?
I just want to be prepared.