She did say that there could still be a disc problem a X-ray doesn't pick up the discs.
This is correct - you can't rule disc disease in or out just with xrays alone, although x-rays can give some clues. An MRI, CT scan or myelogram is needed for definitive diagnosis. You can google "degenerative disc disease in dogs" and find a lot of info. You can treat with pain meds and strict rest based on a presumptive diagnosis of IVDD, esp. if you aren't considering surgery for IVDD (as most people do not in milder cases). I've cut and pasted some things below:
"The results of spinal radiographs can sometimes be misleading. The normal disc, and most degenerating discs, are invisible (as are the spinal cord and nerve roots) on plain x-rays. Only if the disc has calcified will it be evident on plain x-rays. Thus it is common for a disc to displace and produce no radiographic clue as to its precise location. Moreover, remember that disc calcification is common, even in degenerating discs that have not displaced (i.e. are asymptomatic). This combination can lead to an incorrect identification of the disc causing problems. It takes a practiced eye to read through the subtleties and artifacts on a spinal x-ray, and even then the exact diagnosis may be in doubt. Furthermore, obtaining good quality spinal x-rays requires the patient to be completely immobilized, and this usually means employing a general anesthetic."
"How is a slipped disc diagnosed?
A presumptive diagnosis of disc disease is made based on the dog's history of neck or back pain, uncoordinated walking, or paralysis when there is no history of trauma. The physical examination will indicate that the problem originates from the spinal cord, giving further evidence to disc disease. Another important factor is the breed. If the dog is one of the high incidence breeds, the diagnosis is even more likely.
In some cases, plain radiographs (x-rays) may assist the diagnosis, but they may also be normal since neither the disc nor the spinal cord is visible on an x-ray. If the diagnosis is in doubt or if surgery is to be performed, a myelogram may be done. This procedure involves injecting a special dye around the spinal cord while the dog is anesthetized. When radiographs are taken, the dye will be seen outlining the spinal cord. A break in the dye column means that there is pressure on the spinal cord at that point.
Most disc ruptures occur in the middle to lower part of the back. However, they may also occur in the neck. Back injuries often cause paralysis without severe pain while neck injuries usually cause severe pain without paralysis. If paralysis affects all four legs, the disc rupture must be in the neck. Due to the way nerve tracts are arranged in the spinal cord, disc ruptures in the neck may affect the rear legs first and may not involve the front limbs.
How fast do discs degenerate and rupture?
Disc degeneration usually occurs relatively slowly, usually over several days or weeks. The dog often experiences pain and becomes reluctant to move. It may lie around for a few days allowing the body to try to heal the injury, often without the owner being aware that a problem existed. However, discs may also rupture very acutely. Some dogs will go from normal walking to total paralysis in less than one hour."
http://animalhealthcare.com/handouts/dogs/disc.htm
My experience has been that a physical exam by a neurologist or an orthopedic specialist can be especially helpful - much more so than an exam by a GP vet. I have had several geriatric dogs that had degenerative IVDD but none of them required surgery.
Pat