My daughter has a very itchy bum and she scratches it quite a lot. What could be the problem?

Does she still wear diapers? Some diapers have these elastic lining near the thighs for proper grip and to avoid leakage. It could be the elastic or the quality of diaper. Leave her open without diaper for sometime in the day in your presence. Sometimes this can happen if urine is very acidic. So pls see ur pediatrician