Midrash su Levitico 15:28

1) (Vayikra 15:28) "And if she is cleansed of her flow, then she shall count for herself seven days, and after, she shall be clean.") "And if she is cleansed of her flow": (The meaning is) "when (her flow) ceases" (and not "when she has immersed herself"). "of her flow": and not of her flow and her plague-spot. (viz. Chapter 5:1). "then she shall count for her": for herself (i.e., she may be trusted to do so). "seven (clean) days": I might think (that they may be either consecutive or scattered [i.e., with unclean days intervening between them]). It is, therefore, written "and after, she shall be clean": after all of the (consecutive seven days, she shall be clean).

2) R. Shimon says: "and after she shall be clean": after the act of immersion, (when part of the seventh day has passed), she shall be clean. Once she has immersed herself, she is permitted to occupy herself with taharoth (sacred food [and to live with her husband]); but the sages said: She should not do this lest she bring herself to doubt (i.e., lest she see blood after her immersion in the daytime and contravene the count).

(Lev. 15:25:) “And when a woman has had a discharge of blood [for many days].” Thus did R. Hiya teach: Any place where [Scripture] says, “days,” it is two days; and any place where it says, “many days,” it is three days. How so? The woman is unclean from menstruation for seven days and becomes clean on the eighth day. [If] she sees blood on that day, she observes a day [like the rest] and becomes clean. [If] she sees it again on the ninth day, she observes that day and becomes clean. [If] she sees it again on the tenth day, she observes that one and is clean. Ergo, three days which are “many days.” However, if she sees blood on day eleven, which is the fourth day of her cleansing, she returns to her uncleanness, and observes her seven days of menstruation from the beginning. For so have the sages taught: There are eleven days between one menstrual period and another; therefore, the woman has to keep fifteen days. How does she do so? She keeps seven days of her menstrual period. Then after that she counts seven clean days and becomes clean on the eighth day. Then she performs a strict immersion after sunset, and she is lawful for her husband. Thus it is stated (in Lev. 15:28), “And when she is clean from her discharge, she shall count off seven days, and after that she shall be clean.”