Question (R. Yudan): If she checked her garment in the morning and found it Tahor, and in the afternoon she found a Kesem, obviously her garment is Tamei only from when (i.e. right after) she checked it. Is her body Tamei [retroactively] 24 hours?

Answer: You are Metamei her body only due to her garment. Her garment is Tamei only until when she checked it. Is her body Tamei 24 hours?! (Surely it is not!)

(Mishnah): A woman who has relations with Edim (she checks herself with cloths before and after relations), this is like a Bedikah.

Question: How does [reducing retroactive Tum'ah] work?

Answer: If she checked her garment in the morning, and found it Tahor, and had relations at midday, and saw [blood] in the afternoon, she is Tamei only until the time of Bi'ah.

(Levi): Our Mishnah discusses the Ed after Bi'ah, but the Ed before Bi'ah, she is hasty to be with her husband, and she does not check well.

(R. Avun citing R. Ze'ira): Our Mishnah discusses the Ed before Bi'ah, but the Ed after Bi'ah, its appearance changed due to semen (perhaps there a drop of blood, but it is covered).