My dog has been ill and after taking her to the vet found?

They want to do an MRI (cost around $2000) to determine the following: one of these three things could be wrong with her- Intervertebral disk disease, lumbosacral instability or neoplasia (cancer). I am really scared. She has always been a healthy dog and all of the sudden she is sick. They put her on 4 medicines- a muscle relaxer, steriod, pain reliever and stomach acid reducer. She is finally walking again and she is eating all her food and drinking her water. I don't think she would be eating like she regularly did if it was cancer. Any thoughts? By the way, all of this started on a walk when she just kind of fell to the ground and wouldn't move. Nothing out of the ordinary, like a fall, has happened. And the x-rays they took of her show no broken bones or fractures.