First visit with dermatologist
Saw dermatologist first time. He said immediately upon examining with loup that it was contact dermatitis, probably caused by methylisothiazolinone and related chemicals, which he sees all the time in new mothers because it is in a lot of diaper wipes.
I went home and quarantined all products containing methylisothiazolinone. It wasn't in the diaper wipes we were using, but was in the shampoo and conditioner I had been using for past year and a half, and in dish soap I had been using since the fall. Usually I have worn gloves when washing dishes but had been less regular about it since baby was born, so this seemed plausible.
Started stronger steroid, clobetasol, twice daily. Derm said it should clear in a week or two. Made appointment for patch testing.
[While I am less convinced now that methylisothiazolinone could have been the initial instigator, readaing about its possible buildup and neurotoxic effects made me want to shy away from it in general.]