Just an observation....Prednisone also has a warning that it can cause muscle weakness in people with Myasthenia Gravis, but obviously that hasn't stopped the use of Prednisone in the treatment of MG. Just because something comes with the warning that it may cause problems doesn't mean it will cause problems with everyone.

In response to the original question, my understanding is that any exacerbation of MG usually shows up almost immediately and can cause weakness for the first few weeks of starting Prednisone before it evens out after that. Not sure how that translates to the type that you're taking.

One thought - muscle action is affected by electrolytes (sodium, potassium, calcium), and if you're taking the Florinef to retain sodium I wonder if that has put your electrolytes out of balance, and is contributing to the weakness (which is consistent with your comment that you will be having blood work to monitor your electrolytes).