My ds gets that sometimes on the creases of his legs. I put a little powder there and it seems to clear up. I was thinking its just a rash from that area never getting really dry cause of course, even without a diaper, they will not lie down with their legs spread open so the area can properly dry. Even when i dry the area with a towel after baths, it still seems damp.
I also get a similar rash if i dont dry that area of my legs well before putting clothes on. And when i was pg, i got a rash under my big belly if that area wasnt dried well.