CAN U STILL HAVE A PERIOOD 2 WEEKS AFTER YOU THINK YOU MIGHT HAVE CONCIEVED?

IF I HAD SEX AUG 3 AND A PERIOD AUG 12. THEN I MISSED MY PERIOD IN SEPT AND JUST FOUND OUT IM PREG. IS THERE ANY WAY IT COULD HAVE HAPPENED ON AUG 3 AND WHAT ARE THE CHANCES. EVEN AFTER HAVING MY PERIOD FOR AUG