It could be something that she's eating or she could be allergic to her diaper. Try using things that are for sensitive skin and change her as soon as you know she has a soiled diaper.

Use A & D ointment or some other baby rash ointment to keep her skin moist and as a barrier from soil. Since she seems to have more sensitive skin, you could also try using something more natural such as Burt's Bees baby ointment.

I hope this helps.