Purity: Determining the Type of Blood (Uncertainty)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 431:
If a woman knows that the blood being discharged is not blood from an injury but she is uncertain whether it is the blood of istiḥāḍah, ḥayḍ, or nifās, then in the event that the blood is not legally (sharʿan) ruled to be of ḥayḍ or nifās, she must act according to the instructions pertaining to istiḥāḍah. In fact, if she doubts whether it is the blood of istiḥāḍah or another type of blood, then in the event that it does not have the attributes of other types of blood, she must, based on obligatory precaution, consider the blood to be istiḥāḍah.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)