My 6 year old mini schnauzer has been unable to walk very far without his back legs giving out?

I was just wondering if anyone else experienced this sudden onset of symptoms? he will walk and then all of a sudden - bag legs wobble and he is unable to continue - and stops. He does not seem to be in any pain and is eating and everything as normal, aside from this symptom. I will be bringing him to the Vet on monday, but just wondered if anyone had anything like this happen ? I do not believe it to be arthritis as it was SUDDEN, and he is not very old. He's always been very healthy and was just at the vet for a yearly check up last week, all was normal then too.