Good guesses, both!
George's diagnosis is right, although he's missing the name. And if you Google it, you won't find squat. I tried (in case someone was going to cheat), and Wikipedia's very brief reference was just oh-my-God wrong.
The condition is temporary (not PTSD), and known to experienced, well-educated warriors as parasympathetic rebound. It's very important to know it exists if you're running a military campaign. Send troops out to battle and have them engage in piss-your-pants stress (literally) for hours on end. At the end of a day of battle where the body is pegged in the autonomic nervous system red-line (extreme of sympathetic stimulation), the body will whiplash into an extreme of parasympathetic mode. To do otherwise would be to prevent the body from recovering. It has been through the worst it can be in, and now it needs to heal and regenerate - both physically and mentally/psychologically.
Smart military leaders know the phenomenon exists, and might send a wave of fresh troops to attack a few hours after the end of an intense battle. If you don't recycle your own troops, you WILL lose. Your soldiers will not be prepared to get back up to the physical and mental state of being needed to wage war.
Those of us who lived through the 60s/70s and had friends who experimented with drugs understand a phenomenon called "crashing" after being on speed. It's the same thing. One is induced by life, and the other by modern (albeit illegal) pharmacology.
If you want to read more, check out Grossman's
On Combat.
Or... you could talk to a good systems physiologist.

Or... You might even talk to some Chinese elders, or even read some Greek philosophy. Yang needs yin, and all that.
George Mattson wrote:
there are all kinds of battles and not all involve fighting
Indeed. Recognizing hints of the extreme in the familiar better prepares us for the extreme when/if it happens.
- Bill