I let my bf cum inside of me, period due tomorrow. i'm on birth control any risk of pregnancy?

Very minor: Birth control is generally over 99% effective, but not 100%, so there is always a very minor chance. You can increase you chances of not getting pregnant (as well as preventing sti's) by using condoms as well.

Birth control: If you are on birth control then you are already protected. The idea of birth control is to prevent pregnancy from unprotected intercourse.