What Causes Delay In Menstruation Other Than Pregnancy?

ok so i had my last period 6th september 2009 and i had sex on the 10,11,13,17.18,19,20,23,24,25,26,27 and my period was due too come around the 4th october and it didnt so i did a pregnancy test and it came back negative. i waited and repeated the test on the 12th october and it showed positive on a clearblue digital saying 1-2 weeks since conception.

I had my first scan on the 28th november and was told i was 6wks 3 days and my due date was 19th june putting my LMP a 12th september 2009 instead of the 6th september now im 21wks2 days pregnant when roughly did i concieve was it in august xx