Bend and extend the
finger at the
joint slowly to a range of motion possible without
pain. Slowly increase the range of motion. With time the stiffness will go. You can also consult a sports injury center.
Other possibilities are arthritis-rheumatoid, injury, inflammation of the sac of joint, finger prick, insect bite, allergy or contact dermatitis to something, tendon injury or inflammation, clot in blood vessel, infection of the skin or pulp of the finger etc.