Then I remembered that pretty much the same thing happened around this time *last* month.
Aha. All is explained.
When I was in my early 20's (some 20 years ago), I suddenly started getting PMS, but I didn't know that's what it was, because I'd never experienced it before. I found myself taking breaks at work to just sit in the bathroom and sob uncontrollably for no apparent reason. I thought my job was making me so unhappy that maybe I should quit.
When I get PMS without realizing/identifying what's happening, my brain rebels at the lack of context for the extreme emotion, and I unconsciously look for a rational reason. This becomes very confusing, so it's actually a huge relief to realize that it's actually PMS, because then I can just sort of shrug it off, let myself cry if I need to, but not search for some kind of event in my life as a rational source for the feelings.
I stopped getting PMS several years ago (maybe a decade?), but it seems to be back now. In the olden days, I took vitamin B6 and it seemed to 100% prevent the problem (as long as it took it *religiously* every day of the month), so I guess I'll be heading out to the drugstore tomorrow and hope that my body chemistry hasn't changed enough to make the old remedy ineffective.
At least now I know what's up. Bodies are weird.