The only time in the last year that I've had sex was on July 21st-24th. I took a pregnancy test on…

The only time in the last year that I've had sex was on July 21st-24th. I took a pregnancy test on August 3rd and it was positive. Went to doctor he said I'm 7-8 weeks because my last period was on the 14th of July. I'm beyond confused. How is that. And when I had blood drawn on the 24th of August my hcg levels were 52000. I called and she said it was right on for 7 week numbers.