I had sex on 21 august..i am 18.....i started a new pack of ginette on 20....i had my last day of periods on20 of same month....can i be pregnent.???

It's very unlikely..: if you've been taking your pills as directed for more than one month now. If your last period was normal, it would be too soon to pick up a pregnancy anyway. Continue your pills as directed. You should be fine. If you don't get your expected period at the end of this pill cycle, and you're still concerned, you can do a pregnancy test at that time. No worries!