My thumb started acting weirdly… No pain, and I can move it just fine–except that I can’t straighten the last joint (one that’s closest to the fingernail). It can go down and back up, but not all the way. I didn’t hurt my thumb in any way, I just tried to scratch an itch and suddenly found my thumb won’t straighten.
It started a few hours ago and is easing up now, although it still won’t go all the way.
No way to tell without a physical exam. Could be inflammation, could be a sprain or strain, could be nerve damage, could be a tumor, could be a viral infection in the joint…or it could just be “one of those things” that goes away on its own and never comes back.