A spinal injury near the tail should not be life threatening but can be life altering. Serious caudal spinal injuries if untreated can result in urinary or fecal incontinence or both, and paralysis in the hind end. This very serious type of injury would be due to total impingement of the spinal nerves or actual severing of the spinal canal. I think that if your vet had believed that your dog's injury was very serious he or she would have performed X-Rays immediately when you first brought her in. The fact that your vet believed that it was OK to send your dog home on steroids and to wait a couple of days post-injury is a good sign. Even mild spinal injuries can be very painful. Often several analgesics and combinations of analgesics and muscle relaxers must be tried before finding one sufficient to allivate the pain in any species. Additionally, X-Rays may not be enough to pin-point the damaged area; in that case an MRI would have to be performed. Let's hope as Jaybay has said that it is only a soft tissue injury. If she is still in great pain and "not herself" however, I would insist on having the X-Rays immediately and not wait the full couple of days.
Until you get the x-ray results, you won't know if surgery is necessary. Hopefully, this will just be a soft tissue injury to muscle, tendons and/or ligaments that will heal in time. Pain in animals can easily be treated, so call your vet and ask for something. I'm a bit surprised she isn't already on an anti-inflammatory med, but maybe your vet did an injection instead of oral meds? Dogs can take some of the same narcotic meds as people, but these medications are not usually sent home for administration because they can get diverted into the human population.