Quick Version

PEM — The Quick Version

Post-exertional malaise (PEM) in a few short points.

PEM is a worsening of symptoms after physical, cognitive, emotional or sensory exertion.
It can be delayed — sometimes starting a day or more later.
The amount of exertion that triggers it varies.
Symptoms and recovery time vary from person to person, and day to day.
Pacing may help some people organise activity around current capacity, but it cannot guarantee PEM will be avoided.

If symptoms are new, severe, or significantly different from usual, it is sensible to seek medical assessment.

This is the short version. There is more detail if and when you want it.