A little confused

I had my first U/S yesterday thinking I was at 7w6d. The Dr measured baby and put me at only 7w exactly. She said I most likely conceived on 9/20. So here is where I'm confused: the start of my last AF was 8/30 and we did the deed on 9/12. My AF is 28 days like clockwork so I took a test at 29 days and got a BFN. Took it again at 30 days and got a BFP. 30 days was on 9/26. First off I'm a little confused on how I could of conceived on 9/20 if the last time we had sex was on 9/12. Sperm can't survive 8 days can it!? Also IF I did conceive on 9/20 I wouldn't of had a BFP on 9/26 right? Does anyone else think it's possible she measured wrong? Or maybe baby is small? ?