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Hello from JustAnswer.
If that period was on schedule and the sex that you had 3 days before the period was the only sexual encounter during that cycle, it would be extremely unusual for that to have resulted in a pregnancy. The ovulation during that cycle would typically have happened two weeks before that period, and the sex 3 days before the period would have been too late to have fertilized the egg.
It is possible that someone could have what seems like a normal period rather than a missed period early in pregnancy, but there would typically have been a sexual encounter about two weeks before the period for the egg to be fertilized.
If this period is now late and you are found to be pregnant, then it would typically be from a sexual encounter during this cycle, but there also would typically have been a sexual encounter about two weeks before this period.
If I can provide any additional information, please let me know.
You are very welcome. If pregnant, then it would have been from one of the earlier sexual encounters rather than the one last week. However, the next step would be to get a pregnancy test, as there are other reasons for a late period.