Welcome to the merry go round that we call getting a diagnosis. As sad as it is to say, many of us have been through this and it is the WORST place to be.
My advice to you is to first take a deep breath. My second piece of advice is to find a different rhuemy.When you are looking for one I suggest looking for one that treats more than just Lupus. There are 63 AI (Auto Immune) diseases and you already have a couple. Some of the first line drugs for Lupus are not that dangerous and if you have it you may need to be on something. As for the tests coming back positive and then negative that is normal too. These things happening to you are the reason that some of us go weeks, months or even years without being diagnosed.
. If the neuro has looked at the MRI and is sending you to the Rhuemy then it sounds like he doesn't think the legions are MS legions (and we all have things that can appear as "white masses" to the untrained eye. Try to relax a bit but I really do suggest finding another Rhuemy,