Hi 234,
Not every instance of unprotected ejaculation with result in a pregnancy, and you can read about why that is here:
Human Reproduction: A Seafarer's Guide . And no, if someone has their period it's safe to assume they're not pregnant.
I just want to check, because you list your age as 17, do you and your boyfriend have support networks in place, including for things like school, to help you if you do get pregnant?