Can a home pregnancy test become positive 8 days after intercourse

I had a period sept 15, im on a 28days schedule..Me and exboyfriend has unprotected sex on Sept 23, 6 days later (sept 29) me and a friend had protected sex, he was wearing a condom and he "pulled out". 8 days later (Oct 7) i took a pregnancy test and it was positive.  I was expecting my period on Oct 13 or 14.. ofcourse it didnt come because I am pregnant.  But is 8 days too early for the test to be positive even though it was 6 days before my expected period. I am really concerned, who could i have concieved from ?????!!!