It could be a wort. I had one on my ring finger for probably a year.
I had this about 2 years ago, it was very painful. I went to my orthopedic hand specialist and she told me that it was the start of arthritis in my finger and that it would go away. She gave me a small splint like device and told me to wear it for a while because reducing movement in that joint would help it go away. She was right. about two weeks later the pain was gone and the bump is smaller.