It's a play on words. Typically we would use words like "swollen" or "swelling" to describe what happens to someone who's having an immune reaction to an insect bite, as in this panel. "Swole" is a bodybuilding subculture vernacular term for large or muscular, and implies a before and after effect, e.g. "[X] got swole." It's not clear to me if the term "swole" arose from a deliberate misuse of the word "swollen", which is called "catachresis", or whether one person misused the word and others emulated him from amusement. Either could be the case.