I think I've diagnosed exactly what's wrong with my little finger. It's a condition called "Mallet finger". You get it when the extensor tendon at the tip of the finger is damaged. For example, when you catch a ball, but it hits the outstretched finger. I can't remember now when it happened, or how. But it's been like that for several weeks now, and I've not done anything about it, because the human body is mostly self-repairing if you just give it time. But in this case, it hasn't self-repaired.
The obvious diagnostic, is that the tip of the finget droops compared to the rest of the finger, and you can't straighten it. When I compare my two little fingers, there's clearly drooping there.
Treatment is either surgery (I'd rather not) or putting the finger in a "mallet splint" for several weeks.
Hooray for the medical internet. When I had Plantar Fascitis, all my doctor could suggest were stretching exercises, which I did, with no result. But internet research found me the "night splint", and wearing that fixed it in a few days!
So, I've ordered a mallet splint, and a finger stall in case I need that to protect it.
Typing has become slightly more difficult, because I'm now typing with only nine fingers. That might not sound like a hardship for all the two-finger typists, but for me, it's a bit unnatural, and slows me down a bit. I might have to wear it continuously (including at night) for up to 12 weeks. Still, it'll be worth it, if the mallet splint fixes the problem.