Generally when a dog acts lethargic and is shaking it means that they are in pain/ill. If there's no diarrhea and vomiting, i'd just watch him for a few hours. See if he eats, or gets any energy. If he doesn't very shortly, I would say a vet visit is in order. It could be many things.
I to have a dachshund its a mix(terrier)I am fostering her so I have had her for about 4 weeks and at first I thought she must just be scared its a new environment. Then I noticed it when I would talk her stomach would shake while I was talking as soon as I stopped it stopped. That of course isn't the only time she shakes but its when I notice it the most. I'm just wondering if it has something to do with her past? Maybe her last owners would yell at her so she is scared? Your situation could be totaly different? but if anyone has any ideas of why she may shake when I talk please let me know.