MRIs can be inconclusive, fancy term for 'doesn't show it'. LPs too, when looking for O bands. And strickly a brain mri doesn't mean you don't have lesions. MS lesions can appear in the brain, the spine or both. Usually appearing in the brain but that's not always the case.

So you could be suffering from some other neurological disorder, either a CNS disorder or a PNS disorder (in the central nervous system or in the peripheral nervous system) or it could be MS still. Be prepared for potentially having an EMG to rule in or out a PNS disorder, an LP to look for O-bands or signs of inflammation, and/or additional MRI of the spine. It can be a long process, but hang in there, eventually it'll be sorted out .