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.
she could be wiping wrong or too much or too hard after going potty.make sure she wipes gently.