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If she was out in the snow, it is possible she slipped or twisted wrong and possiibly hurt a disc in the back or even pulled a muscle or tore a ligament. At this point it would be impossible to say for sure since she isn't moving on her own. The shivering definitely indicates she is in pain. Dobermans can be prone to disc issues. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. You can read about this here:
Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours. 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. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. Read side effects and precautions here.
If you plan on having her seen by your vet fairly quickly, I would not give aspirin as your vet will likely give a stronger anti-inflammatory medication. If you give the asprin, the vet may not be able to give that medication. If you don't have her seen tonight, keep her crated or confined to a very small area. You do not want her moving as it may aggravate the existing injury.
It does sound like she is in a lot of pain, so a visit tonight would help her pain wise. If you are worried about moving her without anyone getting bit, you can make a muzzle for temporary use in order to get her in and out of the car and vets. Here are instructions on making a temporary muzzle.
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