Purity: Interrupted Bleeding and Determining Ḥayḍ
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 447:
If a woman experiences bleeding for less than three days, and then her bleeding stops, and afterwards she experiences bleeding again for three days, the second bleeding is ḥayḍ and the first bleeding – even if she experiences it during her habitual period – is not ḥayḍ.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)