Hello~I would talk to the doctor about this. It may just be a water-retention issue, I have lost weight in my tummy but my ring size stays the same due to humidity, etc. I think it is worth mentioning.
Are you doing less with your hands than you used to? (Like, have you dropped a hobby that is done with the hands?)
That is very strange. I agree. My fingers reflect what else is going on with me weight wise and don't usually go rogue like that. I lost about 35 pounds a few years ago and was surprised and shocked when all of my shoes were then too large. My shoe size went from 7/7.5 to 8 only after I had my two kids and stayed that way. I had always heard that feet often got bigger do to pregnancy due to the hormones that prepare the body for birth. Just assumed that was what happened. Nope. I am just a chub chub. Hey, I'm back in size 8. What does that tell you. Ha.
Okay, back to your finger. What about temperature changes? When cold, my fingers shrink. However, this would not be the first season for that to happen to you, I'm sure. The process of that is blood vessel constriction which happens to keep your core warm. Could you have blood vessel constriction for some other reason causing the fingers to shrink? Think they call this vasoconstriction. Are you on a low sodium diet? Doing any diuretics?
I do think it is worth mentioning this to your doctor. I hope you let me know what they say!
I can only say that if you lose weight, your fingers will indeed lose fat as well. I have a problem with my wedding ring because I got married while taking a medication that had caused me to gain a lot of weight and when I stopped the med I lost most of that weight quickly and my ring no longer fit.