don't know if I got pregnant before my period

[I'mgaj]

I'm very confused about when I got pregnant. I had sex with a guy on Aug.25 and he (pulled out). I also had sex with my boyfriend that last week of Aug.  On Sept. 2 I started my period. After 3 days of my period I continued to have sex with my boyfried. I missed my period in Oct. and found out I am pregnant. On Nov.1 I had an ultrasound done. The ultrasound said I was 8 weeks and 6 days pregnant and the baby measured avg. 23mm.  I would like to know how accurate the ultrasound is along with the measurements.
If I got pregnant before my last period, then I was really 9 to 10 weeks pregnant. But If I got pregnant after my last period then I would only be about 7 weeks because of ovulation,right? The doctor said I was 8 weeks 5days pregant according to the first day of my last period. When did I really get pregnant before or after my last period? If it was before, could I have been 9 weeks 5 days at the time the ultrasound said 8 weeks and 6days or could I have been less than 8weeks pregnant.
I'm sorry for the confussion but If you could please help, I would greatly appreciate it. My boyfried is so excited, and I realy screwed up by being with that other guy.

[AnnieBrooke]

Hi, Bethaleana, it is not likely.  Do you have any doubt at all that the period August 13-16 was really a period?  If it was, it indicates your prior ovulation was around August 1, and an egg only lasts about a day.  

[Bethaleana]

I had sex somewhere at the end of july and then aug 10th but got my period on the 13 - 16 or 17 and now I am late and haven't had a period in September. Could I get pregnant from having sex the week of Aug 10th right before I got my period?

[AnnieBrooke]

You said, "what I don't get if they give you an ultrasound date is the egg already there the two weeks prior of when the baby start forming."  The way it works is that the medical way of counting out the weeks of pregnancy begins on the first day of your last period.  If a doctor were to say to you "You're 8 weeks 1 day along," he would mean you conceived about 6 weeks 1 day ago.  This is the way pregnancy is counted, not because the egg comes down with the period, but because in the old days it was the only signal they had for starting the count.  

[AnnieBrooke]

I think it is most likely to be the boyfriend's baby, since you had an ultrasound at 6 weeks 4 days and your due date was April 14.  But it's by pretty much the closest possible margin; the other guy does have a chance to be the dad.  The conception calculator I use suggests you conceived on July 22, a number that should be pretty precise since your ultrasound was at 6w4d.  But obviously if it is off by even two days, you can't bank on it.  In your shoes I would go through the pregnancy assuming the baby is from your boyfriend, but would definitely do a DNA test when the baby comes, since it would be unfair to all of you (not the least the baby) not to be absolutely certain.

[Kkaattee]

I meant 2016 on the due dates.

[Kkaattee]

Okay
Last period July 5
sex with boyfriend almost every day before july 23 we live together we constantly have sexual intercorse.
Sex with other guy 24 and 25
No sex with either 26-30
Back with boyfriend 31 sex from then and on.
I had urine test at the clinic 8/13/15 they said I was 5 week 4 days due date April 13,2006
Had an early ultrasound and was about right the dates match I was 6 weeks and 4 days due date April 14,2016