You can get pregnant ANY time you have unprotected sex. As Melka said most women dont ovulate until about 2 weeks after the 1st day of their period but there are always exceptions to this rule and many, many women have gotten pregnant from assuming it is safe to have unprotected sex directly after/during/or before their period.
BTW: sperm can survive for up to 5 days in the right environment.
Usually woman do not ovulate until 12 to 14 days after their period starts - but if you are having unprotected sex and he did stay inside you could be... sperm can live upto 5 days inside.. and if you ovulate early - the timing could be right. You would need to wait 2 weeks since you had sex to take a HPT or maybe a couple of days earlier or later depending if AF comes or not. Good Luck