It is very unlikely you would become pregnant in those circumstances. Google [Calendar-based contraceptive methods] and go for the wiki link. You will see a circular picture of when a woman can be fertile. Remember sperm can live up to 5 days in a woman's uterus. So having sex the day before your period, even if semen went inside, would have the sperm living until day 5 of your cycle at the longest, but that day you are also infertile.
I have but one lamp by which my feet are guided, and that is the lamp of experience.
...
Shall we gather strength by irresolution and inaction? Shall we acquire the means of effectual resistance by lying supinely on our backs and hugging the delusive phantom of hope, until our enemies shall have bound us hand and foot?
Patrick Henry
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