What Causes Sudden Swelling Of Knuckles After Painting?

Hello, my wife is a painter (artistic) and the first two knuckles on her right hand (the knuckles at the start of her index and middle finger) recently became much larger and swollen compared to her left hand after painting for 5 hours. normally she can paint this long and never have a problem, but not this time. We went to the doctor to have an X-ray and he said it does not look like arthritis at all, but simply said it might return back to normal after a while. What could this be? Please note that the swelling is hard and not soft like water retention is. Thanks for any help or suggestions!