I have an unusual round bone on my left hand?!

I have an unusual round bone on my left hand?

I don't have it on my right hand, and it's kind of painful or at least causing discomfort in my hand. What could this be, and how can it be treated ?

Depends on where its located. I have rounded protruding bones on the end of both thumbs.They are rheumatoid changes due to age, but they don't cause much pain at this time. If you are a young person with this type of thing you might get it checked by your Dr. or by an orthopedic physician.