What type of sexual interaction are you asking about? You asked a question about having "sexual interaction" last month, and since you are only 14, I am very concerned about what you are doing. Last month this was with your step-brother, and even though he is around your same age, this is still considered incest and is not a healthy relationship.
You absolutely can become pregnant if you have not had your period yet and are having sexual intercourse. Young women's menstrual cycles are irregular, and the timing could be that you have sex while you are ovulating, and then have your period the first time after sex. If you have unprotected sex while ovulating, you can become pregnant.
Please talk to a school counselor about your sexual health. You need to use a condom EVERY TIME you have any type of naked sexual interaction.
Please let us know more about what you are doing, what you are thinking about doing, and we can find some resources for you.