Many neurological diseases (MS among them) don't have a definitive characteristic or blood test that can tell you for sure that you have them. So they end up doing a process of elimination. Blood work can eliminate things like Lupus or vitamin deficiencies (B12 comes to mind). Evoked potential tests can eliminate determine whether the nerve pathways are functioning correctly and help guide where to look if they are not. MRIs can tell them if there are abnormalities in the brain, spinal taps can tell them if there are proteins in the spinal fluid or other indicators of abnormal inflammation.

When they put it all together and compare it with your symptoms, they may still end up saying they don't know for sure what it is. There are types of brain lesions that don't show up on an MRI. It may truly be from migraines, or it may be sinus pressure from allergies pinching nerves. In the end, you just have to be patient and let the diagnostic process run its course. Try not to pre-diagnose yourself, or you will just cause needless stress over something you can't control. Many of the folks here spent considerable time in limbo wondering what was wrong with us, but worrying won't make it better. So try to relax until you know more from the docs and good luck with hopefully getting something definitive one way or the other out of you latest MRI.