From: Rabbi Moshe Sofer
To: Rabbi Chaim
A woman who immersed one day too early after restarting her shivah nekiyim count and had relations with her husband must be treated stringently. Although the Chasam Sofer explores grounds for leniency—the rabbinic nature of kesamim, lack of established prohibition, and a valid sfek sfeika—he rules that since the neighbor's testimony is clear and the woman does not firmly deny it, genuine doubt is absent; and since she can simply re-immerse, the case is a davar sheyesh lo matirin, for which sfek sfeika and safek derabbanan leniencies are ineffective. She must wait for expulsion of seminal fluid, complete the correct count, and re-immerse.
Source: Shu"t Chasam Sofer YD §156