Really scared

So last year I found out my mother got pregnant at the age of 17 and that i have a sister out there somewhere. I was upset and we haven't spoken about it since and i feel that I can't share things with my mom anymore. Its my birthday tomorrow and i'm turning 18, but two days ago i had sex with one of my close friends, he used a condom of course but i'm still very scared of falling pregnant and i don't want to disappoint my parents because I wouldn't know how to tell them if i do fall pregnant. I'm not on any birth control. I didn't take any plan B precautions and my period starts in 10 days. Should i be worried? My friends say i'm over thinking things and that I should just wait and see but i'm too scared and nervous. What should i do?