There are many causes of diaper rash. It sounds like this is a new brand of diaper that you're using and she may be reacting to the chemicals in the diaper. However, if the diaper was left on too long, it could be an ammonia burn from sitting in urine. If your baby has diarrhea, yeast infection, or illness, it could from that as well. Teething can also cause diaper rashes. :crazy:
It sounds like it would be best to switch diapers if you think it's from the chemicals. Changing the diaper more frequently will help with ammonia burns. Some diaper-free time will also help if you're able to do that. Also, I second what HerbalEarthling said amount using a salve -- especially one with coconut oil. A salve will not only condition your baby's bum, but also create a barrier to help with healing. You could just use straight coconut oil. A lotion will sooth temporarily, but won't create a barrier while the inflicted area heals. Avoid wipes with alcohol and fragrance as that can burn the baby's bum. Additionally, if it's from a yeast infection, do NOT use cornstarch/baby powder as that will feed it and make the condition worse.
I hope this helps you determine the cause and solution!