* Your dog is too cold. Chihuahuas are small dogs with a thin coat, so they can easily get cold. If your dog is shivering, it's likely that he's trying to warm up. You can help him by providing a warm bed or crate, and by covering him with a blanket.
* Your dog is anxious. Anxiety can also cause dogs to shiver and pant. If your dog is new to your home, or if there have been any changes in his routine, he may be feeling anxious. You can help him by providing a safe and comfortable environment, and by spending lots of time with him.
* Your dog is in pain. If your dog is shivering and panting, and he's also showing other signs of pain, such as limping or whining, he may be in pain. If you think your dog is in pain, you should take him to the vet immediately.
If your dog is shivering and panting, and you're not sure what the cause is, it's always best to take him to the vet. This way, you can rule out any serious medical conditions.