Rolling PEM (post extertional malaise) tends to come on quite late after the multiple exertions that caused it, often creeping up on you unseen and then it comes on BIG. Its a trickster because it can even feel good at the time it starts accruing…I hear just so many people saying that they felt so good while they were in the thick of pushing through the big project or doing more exercise than normal or handling the new responsibility at work, that they felt like they were over ME/CFS; that they even thought that maybe they were getting permanently better at last, that it was a sign that they were ready to take on more. At the height of the adrenaline surge, you can feel as though you are more than coping, that things really are improving, that you are putting chronic illness behind you at last, so you then feel more confident to take on more of the "normal" things than you would otherwise dare...until, suddenly, it has the last laugh!
I recently heard a description of it that went along these lines: if normal PEM is a debt that you always have to pay back after the energy overspend, like repaying a bank loan, then rolling PEM is like having to pay back a loan shark with unimaginable amounts of interest added on top. You really don't want to be indebted to that kind of debt collector because it will be utterly ruthless to deal with; there will be no more negotiating or delaying to be had, absolutely no leeway for extenuating circumstances given!