It's one of the symptoms of Chronic Fatigue Syndrome, and it basically means that after you exert yourself, you have a CFS flare-up -- a symptom flare-up. For me, it means getting really tired, or just getting really no-energy.
I had a really good three days the past three days. I got up at a reasonable hour, without too much dragging, made myself breakfast, did yoga, went out to a cafe or, on one day, the library, to do research/writing on my UF novel, walked there and back, made myself dinner, and stayed within my calorie limit (I'm trying to not gain any more weight.)
Today started out the same: reasonable get-up, breakfast, yoga, shower ... and then, yeah, I trailed off. I kept trying to get myself ready to go to the cafe and write some more. The cafe has good salads and that was going to be my lunch, and I sat at the internet and surfed and got hungrier and hungrier. But as I got hungrier, I also got more tired ... until I finally realized that I was having the latter half of a bad day. I considered making lunch but realized I was too tired, so I went to the Mexican place two blocks away, doing the CFS shuffle the whole way.
The CFS shuffle makes me look (in my imagination, I don't really know how I look) like a junkie on the nod trying to walk down the street. Have you ever seen that? Where they're so high they can barely put a foot in front of the other? That's me on a bad day. I'm walking, and my brain is going at close to normal speed, so I can tell that I'm moving too slowly, but I simply can't make my legs move faster.
Usually post-exertional malaise happens pretty soon after exertion. (And it's all exertion, not just physical. Having a two-hour meeting can knock me out for the rest of the day as well. So can having dinner with friends, or writing intently for a few hours.) Generally, the malaise comes because I've used up all my energy with the exertion.
But this time, it seems I'm PEMing for the past three days all at once. Interesting.
Also! I found this article from a lupus sufferer that explains how you have to get through your day when your energy is limited. It's called The Spoon Theory. From a website called "But You Don't Look Sick.com" Indeed.