Can someone help me?

So I’m due end of July I had sex with a guy in October On the 6th possibly the week after aswell I’m not sure I then got what I thought was my period but I don’t know if it was a period or implantation bleeding i then has sex after my period so around 2-11th November then took a clear blue on 23th November and it said 2-3 pregnant I went for my ultrasound they asked when my last period was and give me a due date of 29th July so when would I have conceived October or November and are they accurate