It sounds like your baby affected by yeast infection diaper rash
, which can happen if a mild diaper rash gets infected with yeast. This is especially likely if your baby recently took antibiotics. If your baby's had the rash for more than a few days go-to diaper rash treatments consulting your pediatrician. You'll probably need an anti-fungal cream (there are over-the-counter and prescription versions), which usually helps beat the rash quickly.