I'm not in a position to make a phone call. Beagles are prone to disc issues such as Intervertebral Disc Disease (IVDD). It can occur without injury being involved and can be a degenerative issue in beagles. The trembling and vocalization is likely due to pain. With a disc issue, a dog may move around mostly normal but then move in one particular way that leads to sudden pain. Usually in cases like this a dog will start to avoid jumping and using stairs as the pain increases.
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 this is the issue, I would have her checked over as soon as you can to stop the progression of disease. Read more on treatment here:
That site goes over the condition in dachshunds but it is the same regardless of breed.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.