Probably nothing but my kids once got pinworms from another family, and that makes them scratch their rectal area. If you pull down the back of her diaper in the dark at night and, with a flashlite, look at her rectal area, you can see them. As I recall they are little white, threadlike sqiggly things. The doctor would give you oral meds and everyone in the family has to take it - so much according to their weight. At least, that's how they used to do it.
If it's her vaginal area she's scratching, she might have yeast and as far as I know, you can use monistat for that, but your doctor will know for sure.