I've had my first experience of next-day weakness today, after doing hours of spring cleaning yesterday. It wasn't as strenuous as activities like digging the garden or swimming, but cleaning the oven and all the cupboards inside and out involved a lot of working on my knees and then having to get upright again.
My head, torso and hips have been fine today, but my arms and legs are absolutely destroyed.
On my first trip to the loo after getting up, my legs were so floppy that I crashed into the door frame, then the wash basin, then overbalanced and found myself leaning over the toilet hanging onto the cistern. I was marginally less drunk after a dose of potassium, but basically getting upright and walking have been a nightmare all day. It finally relented at about 8:30pm, and everything's now working as well as it normally does.
If it turns out this is not PEM but an abortive attack of periodic paralysis, it's not surprising the two can be confused, especially without the sore throat. (I haven't had any immune-type symptoms since I started on potassium*.) My arms and legs are weak when I try to use them, sure, but when resting it feels like the profound fatigue and pain that ME patients describe. And the need to rest today has been every bit as non-negotiable as it was before.
I still don't know what's going on, but I'm writing about it in case it helps anyone else clarify what's happening to them. For instance, it had never occurred to me to think about whether there was a difference between how my limbs feel and function the day after activity, vs the rest of my body. As soon as I do, the difference is obvious.
* Re. the sore throats and swollen glands: I'm wondering if mine were to some extent sleep-related. They've always been worse in the mornings, and on a better day would subside once I'd been up an hour or so. My sleep has changed a lot since I started the supplements, and it's possible this has something to do with it. Maybe I was breathing differently or having apnoea episodes, which were worse when I'd over-exerted myself and so lasted all day? Who knows, though.