Diaper rash is one of those things all parents deal with. Some more than others. My little Zoey has a more sensitive bottom than her twin sister Kenzie. It feels like we're trying to fight diaper rash all the time. There are some diaper rash creams and ointments that we've found work better than others on our little girl, but it may not be the same as what works best for your baby. Ask your pediatrician for recommendations.
Something that does apply to all baby bottoms is how to stop diaper rash before it starts. First change diapers often. You probably already feel like you're changing diapers all the time...we sure do when we have three little bottoms to change between my 2 year old and the twins. Pat your baby's bottom dry after a bath to make sure she's completely dry before putting a diaper on. Leave a little breathing room in the diaper, this will help prevent chafing too. Don't use scented wipes, diapers or lotions on your baby's bottom. Let your baby air out, put an absorbent towel or cloth diaper under her and let her kick and play.
All of these tips will help keep your baby from turning red.