Shovevim Shiur Siman 186: A women is trusted to say that she is tamei, but if she changes her mind we don’t always believe her, and she will have to go to the mikveh. What if the women was unsure so she didn’t accept an object from the husband, but then clarified that she is tahor? Why are Ashkenazim so secretive about going to the mikveh, but others are very proud of the mitzvah. Drilling through a water pipe before Shabbos, and when does a wife have to check herself before or after relations.