The determination was: either he jumped up on to/off of something and twisted or bruised something and was just not in the mood to move because it hurt or he had a seizure of some sort. The vet said that all we can do is keep an eye on him. The rule of thumb is 2 or more in a six month period and then they will start to look for a reason.
I would definetly follow up with your regular vet. Keeping good thoughts here .