What could this mystery illness in our elderly dog have been?

[QueenOfFlamingXmasPuddings]

This is going back ages now but we are still mystified as to what happened and thought someone on here might have a clue.

Our lovely dog (Newfie/lab/GSD X, we think anyway) is nearly 13. Last year, we were away visiting friends, and out of the blue he started whining under the kitchen table where he was lying on his bed. DP knelt to see what was wrong, and the dog tried to get up but it was as if his back legs weren't working. DP helped him out from under the table, where dog tried to get to the door but his legs gave out and he had explosive diarrhoea.

He then just lay shivering and whining. It was after midnight and we were away in a very rural area with no mobile coverage to ring anyone, and nobody was sober enough to drive, so DP cleaned up and cleaned dog, covered dog with a blanket and slept next to him on the floor to keep an eye on him and cuddle him all night He stopped shivering and whimpering after about an hour but remained unable to move his back end at all. His legs were basically paralysed, he was dragging them after him when he moved.

By the morning, he seemed brighter, and by breakfast time he was up and walking, drinking, and looking hopefully in the compost bin So, we drove home, and straight to the vet that evening. Vet examined him, found nothing wrong, and basically shrugged and said she had no idea what it could have been as the symptoms had all but vanished by then. She said he was in great shape for his age and size, and to bring him back when it was actually going on if it happened again, oh and to reintroduce food carefully with rice and chicken or lamb for a couple of days.

It has never recurred and nearly a year later he is still going strong and enjoying life, he's getting a bit stiff and sleepy these days but then he is a veteran We genuinely thought that was going to be the end for him though, we assumed it was something like a stroke and were expecting to have him put down in the morning if he lasted the night - it was terrifying for all of us.

So... any ideas?