I'm worried about my dog....please help if you can.

If this seemed to be an isolated incident, he may have just had an attack of cramp in his sleep. Your vet probably did what was necessary and "keeping an eye on him" is the only thing you can do for the time being but I'm surprised that he didn't give you more information about what can cause this. My dog (a tri-colour Welsh border collie with some German Shepherd) began to show symptoms like this when he was about 12 but they were progressive and, unfortunately, due to a brain tumour which was predicted when he had a cancer on the lip. When the vet removed it, he told us that this kind of cancer very often goes to the brain next, so we were forewarned. Your dog is in middle age and if he's otherwise quite healthy and this was just an isolated incident, I think I wouldn't worry unduly. There's the possibility of a mini seizure or stroke (which is why you need to keep an eye on him) but it's much more likely to have been a temporary cramp, especially in the middle of the night. Be aware, though, that if he is under the weather or has a headache or is scared because his legs don't feel right, dogs don't complain about these things the same way that we do but they can become snappy and bad tempered. Especially with a German Shepherd (or any other strong and very owner-centred type of dog), it is worth being particularly cautious about encounters with other dogs and children.