Hi Kathie,
I'm so happy you had such good results with the IVIG. I'm also sorry you're dealing with a new problem.
I'd say at this point knowing what exactly is "wrong" will have to wait until the MRI and any other testing is completed. "Spinal damage" is kind of a vague term without knowing the pathology or mechanism behind the damage. As for pain meds to use, I always prefer and recommend ibuprofen over Tylenol. It is anti-inflammatory and doesn't mess with the liver as much. Lots of folks like Aleve as well.
I have lots of back problems myself, and know what a drag it is. I'm needing to find out exactly what's happening with my back pain, too. Going to doctors just gets OLD after a while.
Just a thought--have you considered that the back pain may be a side effect of the IVIG, since you recently completed a series? I usually have some kind of back/neck pain a few days after an infusion. Since you mention never feeling back pain before, it makes me wonder.
Feel better soon!