I do realize you'll be taking your little one to your vet or a specialist for further workup, but I'd like to post this info for others reading this thread, who may be looking for alternatives:
Consider using a vet trained in VOM:
What Is VOM?
(Especially if nothing comes up on any tests, which does happen occassionally)
VOM Practitioners do treat IVDD, and disk issues, along with other spinal problems.