When am I due?

Okay you guys, so i havent had my period since July 2015 after i took the "Plan B" pill and I thought I was pregnant so I took pregnancy tests constantly and even last week & they all said negative. So i took three Sepetember 21st and they all said positive, me and my husband had sex September 7th and 8th, so does that mean that that was the day I got pregnant? Cause thats the only time it would make since for me to have gotten pregnant. Since I cant go by my last menestrual period since my period is irregular now?