Question:
If a woman’s did a hefsek taharah (Saturday night) but forgot to make a bedika on day one of shiva nikiyim (Sunday) does she need to do a Hefsek Tahara the following day (Monday) or can she just do a bedika on Monday and consider that day 1 of shiva nikiyim?
Answer:
Once she did the hefsek, she doesn't have to do it again, (on Monday) and the bedika on that day would be like day one.
Best Wishes
Sources:
Y:D 196-4