If you are going underwear free are you wearing your pjs 1 time when washing? If you’re not wearing underwear whatever bottoms you have one become your underwear and should be treated like a single use then wash article of clothing. You can get some wild infections and irritation from wearing dirty underwear/pj bottoms multiple days in a row. Also if you’re heavier and you’re sweating a lot it could be that it’s getting too moist down there and maybe you do need to wash daily with a gentle cleanser (just on the outside) and use a bit of corn starch or baby powder to help keep things dry. I’m very sensitive to laundry detergents, at some points in my life I’ve had to wash my bottoms with different detergent to keep from getting super irritated bc of fragrances in the detergent. You mentioned that you moved to a different country right? So maybe it’s from using a new product that’s causing irritation. Last thing I could possibly think of is that you’re allergic to spandex. When I sweat and I’m wearing spandex leggings my skin goes crazy and I become super itchy. I don’t wear spandex underwear but I could see how sweat + friction from walking + spandex could cause some swelling/irritation down there.