Not sure how well you can see it on the pictures! The joint at the top of my index finger has had a solid lump on the side of it it for a few weeks. I don't remember injuring it. It hurts and feels stiff intermittently, can't see any pattern to it, and sometimes it looks more swollen.
My main worry is that its the start of something arthritic, but I'm only 32. My mum has had arthritis since her 40's though, so it's obviously a bit of a concern.
Will see my GP if it doesn't go away soon, but feel a bit ridiculous seeing them about something that is hopefully nothing...