So I laid off the guitar for 4 days. It was unpleasant. Every evening I sat with a guitar, just holding it, which is admittedly pathetic. I felt out of sorts.
We've run through various possible diagnoses and underlying causes. Almost none of them make sense of the onset of symptoms except two: (1) that I suddenly started playing more heavily, and more complicated stuff, leading up to several marathon sessions over a period of about a week, i.e., fatigue and muscle strain, or (2) cutting off circulation for extended periods of time to my left arm, when sleeping, but additionally when walking to and from school, with my bookbag slung over my left shoulder, across my chest and back.
My form, while imperfect, isn't dreadful. I do some weird things with fretting positions, but I've always done that; you can't play minor 7ths with 6ths and 13ths any other way. My habits are awful, but I'll change those ("warming up" doesn't mean playing hot numbers). My posture isn't great either. But none of these really made sense of the pain, because I've always played this way.
My wrist went from stinging with occasional twinges of sharp pain, to feeling mainly tensed with stabs every time I rotated it (at which point I gave up opening doors and turning keys with my left hand, rendering me nearly incapable of either), to feeling more relaxed but weak and sore every time I lifted anything heavier than a fork, to finally feeling basically normal.
Yesterday afternoon I played a few songs on my old Takamine classical. I started with "Norwegian Wood," and then Lauren's song, which I badly missed playing.
This morning my wrist feels ever so slightly tired. We'll see how it goes.