Welcome to HCM,
If you have sex next day of menstrual period chances of pregnancy
are almost nil,as pregnancy depends on Ovulation which normally occur 14 days before expected date of period, in a fairly regular cycle.
Please note if you do unprotected sex and ejaculate out side but pre ejaculation fluid may contain sperms it is very risky to assume that ejaculation inside is not necessary for pregnancy to occur.
Please take precaution and use protected sex that is to use condom as it also protects form sexually transmitted diseases including HIV(AIDS