So, it looks like I have trigger finger.
It's apparently caused by inflammation of the tendon in the finger. I have it in both thumbs, worse in my right thumb but only mild in both (it "clicks" maybe 2 - 5 times per day, that I notice).
This started a few months ago. I've been pretty strict Paleo + high quality dairy (butter, cream, occasional cheese) for around a year. I don't drink very much alcohol - maybe a bottle of red wine per month. I eat quite a lot of oily fish (perhaps 300g / week), no nuts, a moderate amount of fruit, lots of lamb and beef, cream once or twice per week, butter every day, cheese a couple of times per week.
Recently I have quit nightshades to see if that makes a difference. It hasn't yet, after 2 weeks, anyway (removing nightshades from my diet is harder than wheat, for me, too! They're in everything that gives food a bit of life).
Any suggestions? Thanks!