Hi, that's some type of infection + may go away on it's own. One of my dragons developed a bulged toe years ago but it never progressed any further and she had it all her life.
It might also be swelling related to shedding on the fingers. If that's the case the swelling should reduce after the skin on the fingers has shed. Another possibility is that it is a symptom of gout. Either way I'd keep an eye on it to see if the area gets more swollen, if more areas become swollen, or if the finger starts to look wounded.