Had something similar happen. When I wake in the morning I feed the dogs and taking them outside. Twenty minutes later I turned on the pool filter, and out of no where I get this feeling of severe fatigue and then a sensation of weakness in my arms. I noticed also that I was not tolerating the heat too well. I had several ounces of orange juice and as quick as all my symptoms came on, they reversed.
I believe that Prednisone itself can cause weakness, and of course the PMR symptoms. Good luck with everything. Let me know what your outcome is and if your able to pinpoint your weakness. Thanks.