I would like to examine your index finger and look for the range of movements at each joint to detect the probable cause. Rheumatoid arthritis, carpal tunnel syndrome all can lead to pain in index finger.Post traumatic arthritis, osteoarthritis can also cause pain and loss of movements. Painful joints must be allowed to rest. You may need a splint on the affected index finger .If the symptoms are severe consult an orthopaedician.Keep us posted.
Best of luck and regards!