I went to the doctor today and the ultrasound said 8 weeks, and that had conceived the 29th of August.
Edenlove, if the guy you had sex with on August 25-26 had recently ejaculated, he could have had live sperm in his urethra, which could have ridden down his penis into your body on the pre-ejaculatory fluid that all men have when they get an erection. A guy ejaculates millions of sperm at a time, it's not surprising that some of them stay around in the urethra for a while. If the guy who pulled out knows for ABSOLUTE SURE that he had not had an ejaculation within, say, a day, or that he had peed many times in the meantime, then maybe there is a chance you could rule him out. But so many women have written into this site saying they got pregnant even though their partner did not ejaculate, you can't rule out the possibility just because he didn't come. (The odds are lower, just not zero.) If some of the first guy's sperm made it into your body, it only takes one to find the egg.
What you need to do is consider other factors, not just whether he ejaculated or not, but also how regular your cycles are and how long (the whole month -- like 28 days exactly between periods? Or does it vary?) You need to consider your due date is and when the doctor thinks you conceived. Have you had an ultrasound yet, and did the doctor tell you a due date from the information in the scan? (From the crown-to-rump measurement of the embryo, not from the first day of your last period.) A woman with regular cycles every 28 days like clockwork can assume she ovulates around day 14. But a woman with variable lengths of cycles (sometimes 22 days, sometimes 29 days, sometimes 31 days etc.) will not be able to use the first day of her last period to tell her when she might have ovulated. A woman ovulates 14 days before her next period, not 14 days after her last period, so if your cycles don't fit the average length, you need information from an ultrasound to figure things out.