Have you read the classic “Stress fracture in a knuckle” thread? If not it’s worth it just for Stallioni’s Tiggergram joke on the first page but the upshot of the whole thing if I remember correctly was that pretty much no-one turned out to have a stress fracture- it’s much more likely to be a cartilage/ volar plate injury which is what I had on both middle fingers. It takes ages to heal. Barrows had one too and knows a lot more about it than I do from what I’ve read of his posts.
My physio said it wasn’t worth diagnosing as whatever it is the method of healing it (basically don’t piss it off while it’s getting better and deal with the postural issues that caused it in the first place) would be the same no matter what the specific details of the injury were.
People who know more than me will no doubt post on here soon though. I hope it doesn’t turn out to be too bad.