In a dog a stomach twisting is due to eating to fast and not digesting it properly, you need to take your dog to the vets asap, unlike horses the doctor has to go in and try to untwist the stomach. I worked at a vet hospital and saw a case just like it and thank god that the owner brought the dog in asap. However once the stomach twist unless you monitor food intake and water intake it could happen again. But if you haven't taken your dog to the vet please do so, it is very very painful for them.