Joint pain in my left index finger. why?

Possible diagnoses: If the main problem is joint pain with activity and the joint is larger and feels bony, osteoarthritis is a possibility. If there is soft squishy joint swelling and morning stiffness, rheumatoid arthritis would be possible but in RA several joints are affected and usually both hands. Another possible diagnosis is simple tendinitis, usually from overuse. See your primary doc if it is not improving.

See below: Cannot really say without a physical examination. Can be arthritis, tendonitis, joint infection, gout (unusual spot), tendon injury. Physical exam should determine what it is and what should be done about it.