My 9 mo. old daughter gets diaper rash all the time, which is more of the yeast/fungal kind, no matter how frequently I change her diapers, and she gets a bath every single day. It gets red just like that in the folds on the sides. The only thing that works is Nystatin, which you get by prescription. It clears it up almost like magic. (Regular diaper cream doesn't work, except to soothe it a little or to protect the skin from getting too damp and encouraging the yeast rash.)
Your daughter probably does have some soreness, so I'd just be extra gentle and don't use the washcloth there for awhile, and no soap either. Just warm water. Doc still needs to look at her though.