You mention that your daughter is seronegative. Was she tested for MuSK?

In MuSK myasthenia, there is a much less stable response to mestinon. The shakiness you describe may be a disorganized activation of muscle fibers (called fasiculations). It is usually not dangerous but quite inconvenient.

Increased activity can make this worse, while rest leads to improvement.

In some patients a different medication (such as mytelase) may be better tolerated.