myasthenia gravis

For just over 6 months now, I (Female, 31yrs.) have had severe arm and leg weakness and most recently some neck weakness, slurred speech, and sometimes choking with liquids.  I have had numerous tests (MRI's, EMG, RNS, Spinal tap) and my recent EMG showed a neuromuscular junction disorder.  Dr. tested me for AchR and MUSK, but both were negative.  The weakness gets worse throughout the day, and I can do something once or twice (open a door, walk up 2-3 stairs, pick up a gallon of milk) but then can't continue until I rest.  Dr. is going to repeat another EMG, by the appointment time, it will be going on 8 months.  He prescribed Mestinon which has helped tremendously, but I am still not back to "normal."  He stated I do not have classic MG and because the blood work is negative, he cannot confirm it.  I have read a patient's story of MG beginning in the legs/arms, but I still do not have the classic eye symptoms.  Can this still be MG, or suggestive of something else?