Pregnant. How Do I Confirm My Fetal Age? Ovulated After LMP?

Okay, here goes. My LMP was August 17th and I started on Clostil on August 20th (recommended by my OBG). I am now pregnant but only had sexual intercourse with my Partner on September 3,4 & 5. My first pregnancy test were very pale to the point u had to hold it up to the light to see. I visited my OBG on Sept 20th & U/S showed nothing, we even thought it was a false neg. He told me to come back on Sept 27th n there it was a Sac. HE didn t tell me how old I was at the time but based on my cal I guessing I m now around 6wks fetal age n 8 wks gestational but due to the time I had my period and when I had intercourse I didn t know it would have been possible to be preggo. How many days do u think I ovalated for with the Clostil and the fact I got preggo around those dates?? And is this even possible, I thought I would have only ovalated 14dys after LMP??? Please assist.