What kind of staining.,color ? does she go to the bathroom okay not all children wipe themselves well and perhaps some marks from her "rear" end could have come off onto her underwear, expalin to her she wipes from back so it wont go onto her vaginal area, is there any discomfort then she empties her bladder as Ilalos thought, Bladder infection, check these things out if you feel concerned as always a visit to your Doctor will put your mind at rest.