need help!!!

i just had my first ultrasound. i went it thinking i was 11 weeks 3 days according to this app and going off the first day of my last period which was October 28th. i thought my due date was August 4th. going by ultrasound they said August 13th. I am just confused.. This is my 3rd child I asked when i conceived she said around the 17th. but my days app tells me my ovulation was the 4th - 9th. i am so confused someone please explain this better for me.