she could be wiping wrong or too much or too hard after going potty.make sure she wipes gently.
it could be a clitoral adhesion.this is when the foreskin or clitoral hood is attached to the clitoris.you can examine her for this but be gentle as it could be painful.if you see a problem take her to a doctor.it also could be from tight clothing rubbing on her if this is the case looser clothing would fix it.also make sure the soap you are using isn't irritating her.