2,054. Losing the Cloth
Hilchos Issurei Biah 4:17
If a couple engages in intimacy several times in succession, they need not check their cloths after each act. Rather, each party should clean his or her self with their cloth after each act that night and check their cloths in the morning. If blood is found on either cloth, the woman is ritually unclean. If a women was intimate and cleaned herself but then lost her cloth, she may not be intimate again until she makes an internal examination with another cloth. This is out of concern that there may have been blood on the cloth that was lost.
Hilchos Issurei Biah 4:18
Let’s say that the woman put her cloth under a pillow or a cushion and blood was found on it. If the blood stain is long, she is rendered ritually unclean because we assume that it came from cleaning. If the blood stain is round, she remains ritually clean because we assume that it came from the blood of a louse that was crushed under the pillow.