This problem is unlikely to vbe related to the underlying myasthenia problem, or to the medication regime. It may possibly be related to position, but is more likely to be related to position of the lower back than to head position.
Positional changes alone should not cause this problem, so I suspect that there may be a separate problem in the lumbar spine area, this si most likely to be related to a disc problem. The next step would be to have this ara investigated either by neuroimaging of the lumbar spine and / or EMG to see if a sepecific nerve root is implicated.
Further investigations would need to be guided by a neurological examination which should be your next step.
Could you please see my comment below in regards to a reply on "strange symptoms". I am so worried and would appreciate any info you could give me. Thanks!