help! please read

my period is late by 5 days now and i'm starting to get worried. the last time i had sex was june 20th and i got my period july 13th right on time. i was supposed to get it august 13th but it still hasn't come.

i thought i was fine bc i got my period after the last time i had sex but someone told you you can get ur period 2-3 more times and still be pregnant so now i'm really scared? opinions??