How To Calculate The Exact Day Of Conception?

Hi, my name is Destiny. Me and my husband found out we are expecting in October 2014. Prior to that in September we split up. I had sex with another man on September 5th. I then had a period on September 6th-12th. We had gotten back together during that time and had sex multiple time from then until October. It was a normal period. I just thought it was obvious she is my husband s. But I had bleeding when I was 11 weeks pregnant. They did a ultrasound and she measured almost a week ahead of what I was supposed to be. I thought maybe she had a growth spurt. Then on my 20th week ultrasound she measured ahead again. It worries me. Do you think she is my husband s? What are the odds?