Pain and swelling reactions are just that - reactions. Our own body is reacting to an injury or overuse, resulting in Wear and Tear, or has been knocked out of balance and is over-reacting, as is the case in Rheumatoid Arthritis. We can't tell you which it might be but a clue can lie in the symmetry or otherwise of the pain. If the opposite finger is bad too, that would increase the likelihood of Rheumatoid Arthritis. If it is the one finger, did you injure it some time back?
Keep a food diary for a week and see if the pain is worse in the 12 hours following any of these foods - tomato, potato, aubergine, peppers, wheat, dairy or citrus. There are substances contained within these foods that can exacerbate joint pain if not fully destroyed in the stomach.
If the weight-bearing joints are affected, do consider trying a supplement of Glucosamine or collagen. Relief from injury, overuse or Rheumatoid Arthritis can be had by the use of a combination of Arnica Gel which is used externally and Devil's claw which is used internally.