any repetitive motion can cause arthritis, or osteoarthritis if you are predisposed to get it
if they are swollen perhaps its gout … go to your doc & get some anti inflammatories
arthritis is most common in the thumb and its rarely seen in anyone younger than 40 or so. it is degenerative changes at the articular cartilage and it takes years and years and years to develop. rheumatoid arthritis is seen in elderly people and is most common in women and is rarely seen before age 65. it is characterized by the deformity of angulated joints in the fingers so without that deformity i can assure it is not RA. if it was gout you would know it, they would be very red and swollen and extremely painful to move or touch. this is because the uric acid crystals attack the joints causing severe pain. gout is seen mostly in major weightbearing joints like the big toe or the knee and is unlikely to be caused by guitar playing.