Is he teething? That can sometimes change the bm making it loose and acidic. Is he eating any foods that can contain dairy? Parents will first look at the caregiver to blame for any changes in a child. She is correct to ask you, but I hope she asks in a polite, lets work together to solve this problem manner. If you feel you are being blamed and know you haven't feed any dairy, then keep calm face and tone and let her know you are as mystified as she is.
If you can't see any problem why are they making an issue out of it? I would just smile and recommend a diaper cream like baby bum bum which I found works great. I would say she has a concern and is over reacting if in your opinion the redness is minor. Sounds like they want you to feel bad/responsible for this. Don't let them make you feel this way.
Oh and welcome back