Hello, thank you for your question about Arnie.
Shaking can be caused by many different things or could be normal for your dog. It can be a sign of underlying pain/discomfort or difficulty breathing. If Arnie is showing any other symptoms such as lethargy, lack of appetite, vomiting, or diarrhea I would recommend having him evaluated by your veterinarian so that they can examine him and look for an underlying cause.
If Arnie is otherwise acting fine and this is the only change you have noticed it is possible that this is normal for him, and a response when he takes long deep breaths. I would recommend continuing to monitor him if this is the case.
Hope this helps!