My left index finger looks like a clubbed finger. it is very painful and looks as though there has been erosion of the bone on xray. what causes this?

See below: Boney destruction on x-ray is very concerning. Rheumatoid arthritis coming to mind. See a md asap.

Possible Infection: Bone infection can do this and is serious if untreated. If there is pain under the nail a benign tumor called a glomus tumor is a possibility. There are other possibilities including erosive arthritis and you should have it evaluated by an orthopedic hand surgeon. Soon.

Might: Small joint disease in a 26 year old female might be psoriatic arthritis, infection, gout or reactive arthritis. Rheumatoid arthritis doe not cause erosion in early stage of disease. Please see a doctor, and might better to see a rheumatologist.