birth controllll

I'm on sprintec, but this pretty much applies to all oral BC's.
Your period week, the white pills, placebo, whatever you want to call it...since those are hormone free, if you have sex right before that week starts or during that week do you not get pregnant because your body has had hormones constantly for the previous 21 days?
i hope i worded that right. it's not like i'm new to BC or anything, but i was just thinking it last night after sex. i am super freaked out of being pg again and i was like...hmmi just had sex...i start my white pills on sunday...so what if i get pregnant blah blah blah.
im just weird like that....but i'm right about how they work, right? if not...i've had the wrong idea about BC for almost 3 yrs, lol