Niddah 3: Shammai's Leniency regarding Niddah; Contraception

Why would Shammai have been lenient about niddah when he is usually more stringent?  The Gemara suggests a number of possibilities:

• Like everyone, a woman is in a state of chazaka, holding her status, until it is absolutely clear that her status has changed.  Thus it is not necessary to 'back date' her status as a niddah.
• A woman would feel a sensation when her menstruation begins (Hillel states that a woman might mistake menstruation for an urge to urinate)
• Once menstruating, the blood will be seen (Hillel says that a woman might hold the blood in her vaginal walls)
• Rava states that Shammai was concerned that women would continually be concerned about menstruation which would interrupt sexual relations

The rabbis want to know how to determine the beginning of something that cannot be easily detected.  They use other examples to attempt to better understand the movement from a state of ritual purity to a state of ritual impurity.  One is the sota, a woman accused of adultery, who goes through a ritual to determine her status.  Another is what the sota's ritual is based upon: the mikvah.  A mikvah may have lost water but we are not sure exactly when or how that water was lost.  The rabbis agree that either of these examples quite capture the situation at hand.

We are introduced to the question of a woman who is mentally incompetent.  Would she be able to be trusted with self-examination and the reporting of what she may have touched over 12 hours?  It is also mentioned - several times - that a woman might have been using a tightly packed absorbent cloth in her vagina to prevent pregnancy.  These reports are made without judgement; they are matter-of-fact descriptions of marital sexual relations.  How interesting that while contraception was strictly frowned upon, it was regularly used by our ancestors!