Purity: Doubt About Ḥayḍ During Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 455:
If a woman doubts during prayers whether or not she has become ḥāʾiḍ, her prayers are valid. However, if after prayers she realises that she had actually become ḥāʾiḍ during prayers, then the prayers she performed are void, as mentioned in the previous ruling.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)