Many MSers who have had pregnancies enjoy a remission due to the production of estriol which researchers believe has the ability to suppress MS. Estriol is only produced naturally during pregnancy.
If it is not vascular nor can your neuro determine the cause, then, as Nikki said, it is quite possible to be burdened with both fibro and MS.
If you have not seen an MS Specialist, I would encourage you to do so. If your current neuro is an MS Specialist, then perhaps some fresh eyes might see things differently. If you have not run through the Revised McDonald Criteria, I would do so. At the very least it could eliminate any doubts regarding the possibility of MS.