Also after showering her make sure it's very dry. My son is 23 lbs and 7 months so I've had a chunk since he was born lol. Also put some cornstarch or baby powder around her neck. I put cornstarch on all of my son's rolls. He is just so chunky I even have to put it on his feet if not they will smell like cheese!
Take her to her pediatrician most likely just a yeast infection my son gets that they will give you nystatin cream to put on in. Just got to keep it dry always have a burp cloth and wipe under and around her neck to keep moisture out and try to let her neck air dry some is what my pediatrician told and did for my son
I'm almost certain it turned into a rash due to not cleaning her double chin as often(not saying you don't clean her) and hitting something with only water doesn't make it clean! There gonna give you s cream for the rash and info about bathing her properly since she's chunky! I'm sure not too serious