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It sounds like the shaking and discomfort might be caused by pain. If you know that there was not any significant trauma, you can assume that it might be arthritic pain or something else that is causing this.
I suggest that you see the vet as soon as you can. This might be a small problem that is fixable today and it might not be tomorrow. At least, the vet can give him some type of anti-inflammatory to help with any pain he might be in.
Let me know if you still have questions.
Arthritic pain would not come on suddenly but it could be worse on certain days. The weather and temperature are big factors in arthritic pain.
For the shaking, I suggest that you put a sweater on him or give him lots of blankets to to burrow in just in case he IS cold.
There could be any number of possible scenarios. I am sure that your vet can give you a better idea because he can actually evaluate the dog.
Based on my experience, shaking is usually caused by pain, being cold or being sick.