You did not mention if this is a new reaction or if you got the dog this way. It could be many things like abuse or a fear of something it smells or senses. Be sure to get her/him into the vet ASAP. It may not even be scared - just very ill.
Shaking is a sign of pain. Get your dog to the vet!
My dog is having sorta the same problem. He is a large pitbull/mastiff mix. We think and he is shaking every few minutes. Like he`s cold. But our house is to warm. He has no energy when he used to be so hyper we all couldnt stand it. It just started last night. I don't have enough to take him to the vet.
Something might b wrong my best advice is to please take the dog to see what's going on and don't wait because the more you do the worst it might get!!!
We found a dog and he is really weak, yelps when you touch him near his tail, has his tail tucked down and keeps shaking and shivering. I am not sure what is wrong with him, he isn't getting up much. But can walk around slowly and is eating.