When describing the ritual impurity associated with a nidda, the Torah teaches in Sefer (15:19) “And if a woman have an issue, and her issue in her flesh be blood, she shall be in her impurity seven days; and whoever touches her shall be unclean until the even.”
This passage is understood to mean that there must be an actual flow of blood that is accompanied by a physical sensation. For this reason, if a woman were to discover a ketem (stain) on her clothing, according to Torah law she will not be rendered a nidda. Nevertheless, for a variety of reasons, the Sages of the Mishna ruled that in such a situation, when a ketem is discovered on clothing, the woman is deemed to be a nidda on a Rabbinic level.
The Gemara on today’s daf discusses the relationship between a ketem and the actual flow of blood –
Our Rabbis taught: If a woman observed first a bloodstain and then she observed a discharge of blood she may for a period of twenty-four hours ascribe her stain to her observation; so said Rabbi [Yehuda HaNasi]. Rabbi Shimon ben Elazar ruled: Only during the same day.
According to Rabbi, the woman’s state of ritual uncleanness does not extend retroactively to the time the article had been washed but begins at the time the stain was found, as long as the two events took place during a single 24-hour period. Rabbi Shimon ben Elazar agrees that the two events can be related, but limits that ruling to a situation where both took place during the same day.
In his Ḥokhmat Betzalel, Rabbi Betzalel Ranschberg points out that there is no clear ruling with regard to this question and that none of the codifiers relate to it in any way. He explains that according to the current custom this question is not a practical one, since the ruling of Rabbi Zeira that women always wait seven clean days after seeing any spot of blood.