BTW: You will get more responses if you pose your question in one of the threads. This is for blogging...and not everyone reads it. You could post in the general ME discussion or on a more specialised thread. Good luck!
PEM stands for "post-exertional malaise", considered by most (I think!) to be the hallmark of ME/CFS. It happens in response to exertion, whether physical, mental or emotional. It's an abnormal response., it's usually delayed onset, often occurring 20-24 hours post-exertion.
I agree with hellytheelephant, that you would get more responses if you post this in one of the threads.
Your weakness could be from orthostatic intolerance, PEM, lack of potassium, lots of things, so it's helpful to pay close attention to what, if anything, you were doing or any supplements etc. that you took before the onset of muscle weakness. Another very possible cause is adrenal-related fatigue - I was weak as a kitten with this several years ago. A good integrative or functional medicine doctor or perhaps a naturopath may be able to help you sort this out. All of the things I mentioned above have wiped me out from time to time and it's important to find out the cause.
And, good doctors being very rare, another thing you might consider is muscle testing done by a competent chiropractor. Mine helped me a lot with several issues, including my adrenals.