This could be a symptom of Rheumatoid Arthritis, but it is best to see your doctor about it to make sure.
My mothers fingers does that all the time you may have carpal tunnel syndrome but you sould check with your doctor though
Lack of potassium,arthritis,overworked muscles/tendons.
It might be a disease of the agility of the fingers. Contact a doctor immediately!
Its nothing your finger are double jointed so thats why.