I'm so sorry to hear about your little girl. I know how scary it can be when your pet is ill and in pain.
I agree with your vet in that this could be a back problem. An MRI would really shed some light onto the situation, as this diagnostic test clearly shows the patient's soft tissues, in addition to the bones, so it could really help to shed some light as to what's going on with your little girl.
But another possibility to consider is some sort of neurological pain disorder. If this were the case, you wouldn't necessarily see anything abnormal on an x-ray or blood tests. If an MRI and today's vet visit doesn't lead to a conclusion, this is a possibility that I would seriously consider.
A veterinary neurologist is a specialist who I think may really be able to help your little dog. Here's why: neurologists specialize in the workings of the brain, spinal cord and nerves. If there is nothing wrong with the mechanics (i.e. bones, muscles, tendons, etc.) of your dog's body, this means that the problem is probably the result of a problem involving the brain, spinal cord or nerves.
The nerves perceive sensations, like pain, and they transmit these messages along the spinal cord and up to the brain. Some nerves go directly to the brain too. So if you have a problem with one or more nerves, you can get scrambled signals, incorrect interpretations of sensations, and other mishaps. It's possible that a nerve was damaged and the nerve is incorrectly interpreting the signals it receives as pain, so messages of pain are transmitted to the brain and your dog experiences sensations of pain.
I should also note that abnormalities with the spinal cord - from things like injury, spine abnormalities and certain cancers - can also result in these "scrambled" messages, and so your dog could be experiencing pain when there's nothing painful occurring.
The problem could also be at the receiving end of the messages - at the brain. The nerves could be working properly, but the brain is not interpreting the signals properly and this is causing your girl to experience pain or tingling or other odd sensations that cause discomfort.
Now, there's all sorts of things that can be done for your girl. There's medications and other treatments that may be able to help her if her problem is neurological in nature, so I would definitely recommend seeing the specialist. Many larger, 24-hour vet clinics and referral hospitals have neurologists and other specialists on-staff, so often, the neurologist can employ the expertise of another specialist to help in finding a diagnosis.
A neurologist or another veterinary specialist can perform all sorts of tests with the brain, nerves and muscles to determine exactly what's going on. These specialists are also more versed in some of the more uncommon disorders affecting pets, as they specialize in a particular body system, whereas your normal vet is likely more of a general practitioner, who is versed in a diagnosis of the more common afflictions affecting our pets.
I hope you find a diagnosis and effective treatment for your girl very, very soon. I'd also love to hear an update on how she does.
Don't hesitate to let me know if you have any additional questions!