Pesachim 81A

בשני שלה, ואחר כך ראתה ⁠— אינה אוכלת, ופטורה מלעשות פסח שני.

on her second day, after she immersed in a mikveh. At that point, it is unclear whether she will remain clean of discharges for the remainder of the day, in which case she was pure from the time she immersed and may eat the Paschal lamb at night, or whether she will experience a discharge of blood during the day, in which case her immersion is retroactively invalid and she was impure the entire time. And after that, she saw blood, thus retroactively clarifying that at the time the Paschal lamb was slaughtered she was unfit to participate in it. The halakha is that she may not eat from the Paschal lamb due to her ritual impurity, but she is exempt from performing the ritual of the second Pesaḥ.

מאי טעמא? לאו משום דמרצה ציץ? אמרי: לא, משום דקסבר רבי יוסי: מכאן ולהבא היא מטמאה.

The Talmud explains: What is the reason that she is exempt from the second Pesaḥ? Is it not because the frontplate appeases God, and therefore her first Paschal lamb was valid? Consequently, it is clear that the frontplate does appease God for uncertain ritual impurity related to the discharge of a zava. Say in refutation of this proof: No, this is not the reason. Rather, it is because R' Yosei holds that she renders objects impure from now and onward. When a woman who keeps watch a day for a day experiences another discharge on the 2nd day after she immersed in a mikveh, she is not retroactively considered to have been impure the entire time; rather, she begins a new period of impurity from the time of her second discharge. Therefore, when the Paschal lamb was slaughtered on her behalf, she was ritually pure.