Yes, you can have MS without any lesions showing up on an MRI yet... did they do the MRI's of the brain and spine with and without contrast? This helps to be sure they see anything and everything that may be there. Doctors use the tests to confirm a suspicion mostly but rely on clinical evaluation of symptoms pretty heavily given it can take time for the lesions to present themselves. They do spend quite a bit of time and testing to try to rule out anything else it could possibly be .
There are some posts back on 11/3 that deal with this question you may want to read. Several people give good descriptions of this subject. I don't remember whether you have said whether you have had a spinal tap yet or not. Typically that is one of the first things they do.
I know it can be so very frustrating. I am sorry you are having to go through all of this when you just want a straight answer. If they did not do the MRI with/without contrast, you need to insist it is done again. That is the most reliable way to do them. You may need to see several neurologists before you get the answers you need.
Lisa