From: Rabbi Moshe Sofer
To: Rabbi Shmuel
A woman who experienced sudden fright had relations that night, and blood was found the next morning. The Chasam Sofer rules that sudden fright produces an immediate, noticeable physiological reaction, so if the woman felt nothing at the moment of fright there is no obligation to check beforehand; the blood found afterward is attributed to an unrelated cause, and the couple retains their status as anusim and does not require kaparah.
Source: Shu"t Chasam Sofer YD §163