These are also typical symptoms for tick disease, or things like toxoplasmosis or neospora. We do not have tests for all of the tick diseases out there, and some tests will yield false negatives, so diagnosing can be tricky. If you saw a quick response to the abx, that is very telling to me. If this is the case, I am willing to bet an MRI will not be helpful.
What antibiotic were you given?