Welcome, Rach! Your story is one that is very very familiar to many of us. I was fortunate in being originally diagnosed by an MG specialist with a postive Tensilon test. But in recent years I've had 2 neuros try to take my diagnosis away because I'm seronegative and had a negative EMG. I've never had the single fibre EMG, not have I been tested for other antibodies. Though from what Lizzie posted after seeing her neuro, it is possible to have negative antibodies one test, postive the next, then maybe back to negative. If you do have MG, it can be extrememly difficult to diagnose. Neuros tend to want us to all present exactly alike, but I don't think any two of us have exactly the same combination of weakness, antibodies, test results, etc.
One piece of advice I would give you. Start tomorrow, and try to REALLY wear yourself out. I mean, to the point of exhaustion. That way all of your symptoms will be at their worst when you see the neuro on Monday. Whether it is MG or something else, give your neuro as many evident symptoms to work with as you can. The good thing is, if you can get a diagnosis of MG, or at least a sympathetic neuro who realizes there is something wrong, though he might not be able to put his finger on exactly what it is, that will go a long way toward helping you on your appeal for SSD.
Not that I hope you turn out to have MG, but I know that for you, right now, having a firm diagnosis is better than not knowing anything.
hugs,