Under-skin infection may be from staph or strep.
Infection usually invades
under the
skin if there is any (even slight)
skin injury, fissure
If it was contact dermatitis (soap, new clothes or washing detergent), there wouldn't be under-skin fluid, I guess.
It may help, if you check in google using "staph scrotum", "celullitis", "staphylococcus groin", "streptococcus groin" and related terms.