If you wear tight clothing around you waist/hips. it could be related heat rash or sweat that can't evaporate and causes rubbing on the hair follicles in the area.
My sister had this for years before she realized it was from the rivets and zipper on her jeans. She is very allergic to nickel (has always had to be careful about jewelry) but never considered that there is often nickel alloy in rivets and zippers!!!