Late ovulation help please

My first day of my last period was August 20th . I got my bfp on September 18th it was very faint so I did blood test and it was positive too . now i should be 7 weeks and 1 day . When i went to the ultrasound appointment she told me the baby is 4 weeks only and there is no heartbeat yet . She said you maybe had late ovulation !! How i would get positive before two days of my period if it was late at ovulation? anyone experienced the same?