when did i get pregnant

i dont remember my last period or what the date of my last missed period wouldve been , &i got my first ultrasound on 11/2/11 and it showed by measuring my babys head stomach and leg that iwas 14 weeks&6days . (looking back .. that would be 7/21 when i conceived) .. but i didnt have sex that day , am i calculating wrong ? the first time me & my current bf had sex was on 7/29 , and the last time i had sex before that was the 4th of july, (with an ex) . idont understand . the first time me &  my boyfriend had sex (7/29), it was unprotected & i firmly believe he came in me. somebody please explain this to me .

[RockRose]

If you were 14 weeks,  6 days pregnant on Nov. 2,  that would mean they're assuming you started your last period around July 21.  So that would mean you got pregnant,  by their calculations,  around Aug. 4