From: Rabbi Moshe Isserles
To: Unknown recipient
The Rema discusses a case in which a woman, for purposes of concealment, falsely declared herself tame’ah before her household, and her husband now asks whether he may be with her in private since she is in fact tehorah. He analyzes whether this creates a binding status of huchzekah niddah bishchenoseha that would prohibit her to her husband, noting the opinions that treat such a public presumption as strong evidence. However, he concludes that since her statement was made knowingly for external reasons and is reversible through a credible amatla, the principle of peh she’asar hu peh she’hitir applies. Accordingly, the Rema rules that she remains permitted to her husband in private, as the false declaration does not create a definitive halachic status of niddah in this context.
Source: Shu"t Rema §2