It is pretty common for early toddlers, once they are eating solid foods, to have diaper rashes in any areas that poop comes in contact with. According to my child's pediatrician, this is usually due to a high acid content in stool that can irritate the skin. It can sometimes be related to diet, but isn't anything to be very worried about.
Management of ...
Wow, you are almost literally in the same situation my wife and I were in about a week ago. Over five days we
applied Desitin Maximum Strength to the butt rash each diaper change,
gave her an hour of diaper-free time in the morning, and
gave her a bath at night with Aveeno Baby Soothing Baby Bath
It took a few more days after for the rash to go away ...