What Is The Remedy For Itchy Bottom?

My two year old won't stop scratching her bottom, I think that it started when we put her in pull-ups, and when I notices the raised skin that she was scratching and tried to pin point what had changed we put her back in diapers. It hasn't completly gone away and I am not sure how to get rid of it. I tried another type of pull up with the same result. what can help?