we had this same exact thing happen it is completly normal and has to do with the beginning of teething. we switched to a rice cereal for this which seemed to help a little and also used the triple-paste diaper rash gel. it helped a ton with the rash. nothing to worry about though as i said this is normal and will pass as hard as it is watching your baby scream when you wipe them with this terrible rash.
how to treat diarrhea in a 6 month old baby who has had over a week and his bottom is raw. my daughter has taken him to er and walk in clinic twice