Help Clarifying Conception/Due Dates?

I took a pregnancy test the weekend of June 11-13th. I took 3 tests and they were faintly positive and not taken in the morning. I went to my dating ultrasound June 23 and I was told I was 6 weeks and 3 days. I would like to know if the ultrasound could have been reading wrong and I actually was 9 weeks and 3 days? Not only that, I have two possible conception dates April 25 (sex on April 18th) and May 23 (sex on May 21) which one should be more accurate?