AB, your giving a pretty good description of PEM. It's always been very hard for me to determine boundaries because they are not constant. I too feel better during exercise, but it's what comes after that sucks.
I despise the word "malaise" because it doesn't come anywhere close to describing what it actually feels like. I like the terms "Post Exertional Relapses or Crashes" because for me it's basically an exacerbation in severity of the usual symptoms. I would describe PEM as feeling poisoned. Physical exertion is by far the worst. Mental or emotional stress is next. In the past, infections or getting too hot would cause crashes as well. No longer have those.
I used to have delayed onsets, but that's not always the case now. My PEM has changed in how it presents based on the level illness I'm in at that time. At my worst, it took very little to set it off, I would have up to 24 hour delay onsets, it would take weeks, and at times months to come back out of it. At my best (currently), it takes a lot more stress to set it off, and when I do it most often hits me right away and is not completely delayed.......and I will recover that same day.
It can also be cumulative. The amount I do today may not be too much....the same may true for tomorrow and the next day doing the same amount.....but then on day four doing the same amount, I will crash.