Abcess, AKA saddle sores. Sometimes people just explode, spontaneous human combustion.
Could be caused by an ingrown hair, infected zit, any number of relatively gross but not abnormal things. Best advice is to treat it before it gets that bad. Ask your doctor what to do when you feel it coming on.
In addition to washing the gear regularly it's good to have a couple different brands, styles of shorts and swap them to move the irritation.
I've never been "blessed" as you but know guys who have and these are things they've discussed.
Ron