Sore bumps occasionally appear on penos and in groin area?

occasionally, maybe once every few months, I get a small bump somewhere in my lower region, usually inner thigh or on my penis shaft. these bumps grow up to a certain size, then slowly shrink down and disappear. does anybody have a clue as to what they are? they never really bother me or cause discomfort, so I havent gone to a doctor about it, but I’m curious as to what it might be. I’m 20, and not sexually active, though I have a partner who is starting to want to be, and if it’s something that can spread/ is infectious I would like to know.

Just to restate, they dont only appear on the penis. However they do seem to grow larger when they are in locations like my thigh. I also share towels with my family members, so I’m pretty sure whatever it is came from one of them, if possibl.e.!

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You need to get your own towels and keep them in your own room so that no one else can use them to rule out the possibility of cross-contamination