According to the Mishna on our daf, if the sota is in fact guilty of adultery, after she drinks the “bitter water” her face will begin to turn green and her eyes will bulge out. At that point the people standing nearby immediately remove her from the Temple precincts lest she me tameh the holy place.
The Gemara attempts to clarify what the fear of ritual defilement might be. It cannot be a concern that she will die since – at least on a biblical level – someone who is tameh met (one whose ritual defilement stems from contact with the dead) is permitted in the ezrat nashim – the women’s courtyard – the area where she is given the “bitter water.” Abaye explains that the concern is that she might bleed and become a nidda.
The Gemara offers support to the idea that a sudden fear might cause a woman to become a nidda from the passage in Megillat Esther (4:4), which is understood by Rav to mean that Esther became a nidda upon hearing that Mordekhai was in sackcloth following Haman’s decree. At the same time, the Gemara questions whether this is true, given the Mishna in Massekhet Nidda which teaches that fear stops a woman from menstruating. The Gemara’s explanation is that although a long-term fear may keep a woman from menstruating normally, a sudden shock may cause a woman to bleed.
A woman’s monthly menstrual cycle is dependent on hormonal activity which is directed by the brain. Severe emotional stress, like a long-term threatening situation, may cause regular menstruation to cease – even for an extended period – until the stressful situation has passed. At the same time, a sudden shock or severe emotional event may cause a woman to bleed outside of her normal cycle.