Well, I believe you have to be on the pill atleast a full 7 days for them to become effective, though I want to say that it could possibly be an entire month before they become fully effective. If you have regular 28 day cycles then you would have ovulated around or on the 27th of august, right after you had unprotected sex. Since sperm can live in your body up to 7 days, with the average being about 3 or 4 it is very possible you could have become pregnant. I would recomend you wait until 14 days after you last had sex (around the 9th or september I believe) and go take a pregnancy test. Also, there has been no research to show that taking birth control pills has caused birth defects, but you should stop taking them if you find out you are infact pregnant.

You need to be on the pill for 7 full days for it to be effective..

You can get plan b, the morning after pill.. It is approved for 5 full days after unprotected sex, you could get it from planned parenthood if you need to =)