Good question...I was always told yawning expresses excess co2 from lungs. If true, then maybe with weak pulmonary function because of MG weakness could account for the weak feeling followed by the yawning.

The real issue is, How long does the weakness last and do you ever get the feeling something is sitting on your chest...or a feeling that the effort to take a breath is almost not worth it?!! If so to either, be very careful about a crisis event.