in my experiance, fingers tend to swell with climate change, like when you suddenly go from indoor- heated areas to outside in the cold. or vice-versa. i think its just a reaction to the change in temperature, sort of the way that roads get potholes. fingers in general tend to swell depending on blood-flow, and it has to do with circulation, so if your arms are held downwards for long periods of time, your fingers will be more swollen than if you held them above your head. if youre worried about it being harmful or something, i dont think that's the case, because nobody has ever told me it was bad or that there was something unhealthy about it, and its been happening to me since like middle school.