I think if you get sore throats, swollen glands, flu like feeling or just feeling sicker, rather than just tired or exhausted, you have PEM. If you do not get that it doesn't mean you don't have it, things could be going on under the surface.
Interesting.
maybe OT, maybe not:
I definitely have PEM, but not the first two and depending on one's definition, not the third. I do get exhausted, KO'd after any form of activity outside my capacity, which is variable. Often I'm out cold about 4 hours later, exhausted (more than baseline) one day after and still exhausted, nonfunctional two days after, not improved. Total brain fog, heavy limbs, etc. Recovery to pre-activity can take a few days, weeks, sometimes months or as I've learned at great cost, it may never fully happen.
Activity for me at least doesn't mean specifically aerobic exercise, it seems to mean (anything that requires a physiological response) x (time of exertion) x (time away from bed). Standing in one place is
much worse than gently pedaling a bike for the some amount of time, although it "less effort".
For a long time I thought that's what everyone meant by PEM.
I think we often use the same words to mean different things, probably without realizing it. In the local mecfs support group, there are several people who report not having brain fog. I had believed that was a required symptom; if you don't have brain fog you have something other than M.E or CFS. I didn't know what to make of that, still don't.