Cin - the answer is that this is quite likely an MS HUG - your description is classic. this is due to a spot of damage on the thoracic spine. A lesion on the thoracic spinal cord. Period.
So what if no "MRI spots" were seen. Not all the spots of damage we have in MS are imaged on an MRI. NO neuro should tell you that an MRI sees all there is.
The difference here is in the use of the word "lesion".
MS causes lesions. All symptoms that MS causes are caused by areas of damage in the CNS. These areas are lesions. These are the REAL lesions.
The MRI shows the larger areas of damage as spots on the image. These are called lesions, too, but they are not ALL that is going on in the nerves. they are just what that machine was able to see. I call these "MRI lesions."
When I developed Trigeminal Neuralgia which is caused by an attack on the Trigeminal Nerve which comes off the brainstem, we did a new MRI. My neuro, who was very good, said, "Well, I don't see anything on your brainstem to acccount for the Trigeminal Neuralgia. That's good! Nobody needs a big lesion on their brainstem." What he was saying that by having the pain, he knew there was damage on the nerve or brainstem. When it didn't show up on the MRI, he also knew that it was just too small to be visualized. Your HUG is the same way.
You might want to read the Health Page called Lesions vs Symptoms
http://www.medhelp.org/health_pages/Multiple-Sclerosis/Lesions-vs-Symptoms/show/61?cid=36
Does this help?
Quix