Due date

I just came from my first doctors appointment, my doctor tells me I'm 8 weeks or more, and gave me a due date of Aug 20th which means I conceived this baby November 28th which is impossible I was on my period due to a miscarriage that I know I had because I went back AND forth to the hospital to get my levels checked. Then she goes on to tell me I prolly never had a miscarriage AMD that I still my be pregnant from November. I had sex one time during my fertile window which was December 16th which gives me a due date of September 7th meaning I'm 5 weeks & 5 days I don't believe I'm 8 weeks or more however I won't find out for 3 more weeks until I get a ultrasound. I know I'm not a doctor but what I do know is my body BTW we made a bet she's going to lose and she'll have to pay for my stroller! Have anyone else experienced this?