Purity: Uncertainty of Blood Source (3-10 Days)

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 443:

If a woman experiences bleeding for more than three and less than ten days, but she does not know whether the bleeding is from a boil, wound, or due to ḥayḍ, she must not consider it to be ḥayḍ.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)