Hi & Welcome.
I can understand your problem.
Yes, although less common but it is possible.
Since the fact that you had unprotected sex on her most fertile period (on September 10th) and the pre cum contains sufficient number of active and motile sperms capable of swimming and fertilizing the egg inside the
uterus so the chances of
pregnancy although less but it can not be ruled out it.
Other possibilities of her delayed period could be
stress, emotional disturbances,
hormonal imbalance or disorders, certain underlying medical illness, change of place, etc.
So, it is better to get her home pregnancy test done if her period is delayed for more than a week or so and consult a gynecologist.
Hope this answers your query.