We usually see that type posture with back issues and abdominal issues. Given that your dog is a dachshund and they are prone to disc issues, that is probably the cause in your dog. The trembling is likely to be from pain. There are other causes for trembling such as hypoglycemia (not applicable since your dog is eating) and seizures (usually don't last very long). Dogs will also tremble when stressed as well. I'm going to give you a site that goes over IVDD in dachshunds.
Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours. I know you used it, but I want to make sure you have the proper dosage. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. Read side effects and precautions here.http://www.petplace.com/drug-library/acetylsalicylic-acid-aspirin/page1.aspx
Since your dog is in pain, I would have him seen as soon as possible.