Purity: Obligations of Slight Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 392:
For slight istiḥāḍah, a woman must perform one wuḍūʾ for every prayer and she must wash the outside of the vagina with water if there is blood there. And based on recommended precaution, she should purify the piece of cotton [or sanitary pad/another absorbent item] with water or change it for every prayer.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)