He may have gotten into something when you weren't watching. I can't diagnose your dog over the internet and I am not a vet. The only person who is going to be able to give you an answer to this question is a veterinarian. I would take him to an Emergency Vet today!
Take Dog to Vet asap! I had the same problem with my dog and she needed surgery asap!
I strongly suggest a Vet!
I strongly suggest a Vet!
My dog just today was playing and having a good day all of a sudden his back feet just dropped we took him to the vet and we had to just put him to sleep don't wait get there A.S.A.P.