She absolutely can get pregnant the day before her period. Women can get pregnant any time of the month. That means we can get pregnant before, during, and after our period.
In a typical 28 day cycle, ovulation (when the woman is most fertile and most likely to get pregnant) happens around day 14. However, few women are like clockwork. That means our cycle might not be 28 days long, and we might not ovulate at day 14. Some women even ovulate when they are still bleeding from their period. Also, sperm can live for a few days inside the vagina. That means you could have sex a few days before she ovulates (becomes fertile), then that sperm could still be in her vagina a few days later when she ovulates resulting in pregnancy.
It is very difficult to tell when a woman is ovulating. Most women have no idea when they are. So protection needs to be used every single time no matter what unless you are trying to get pregnant.
The bleeding she is experiencing is probably her period or vaginal tears from being too dry during sex. If you two had sex for the first time tonight, it would be too soon for her to have implantation bleeding. If you two have been having sex before that, then it could be implantation bleeding. The only way she can know is to take a pregnancy test.
You can check out this site for more sex, birth control, and fertility related questions:
http://www.coolnurse.com/sex_faqs_index.ht
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