Update: turns out this was a prelude to a diagnosis of rheumatoid arthritis, which affected both hands, my right shoulder and neck, and exacerbated a knee problem.
Thankfully, the doc reckons it has been caught early enough to be contained. I've been prescribed long term meds, and had an injection of "something" yesterday that has had dramatic results.
I've been getting physio for the knee and, after about 6 weeks off the bike (just a coincidence of timing with lockdown), I've managed two epic rides of 11km over the past couple of days with no adverse effects. It felt great to be out again.
I see there are a couple of threads here about arthritis, so I might just join in there.