Help?

Gemma

My last period was 19th - 21st Sep. I got a negative pregnancy test 15th November then a positive test on Monday (26th). I know I had sex on 4th November. The maternity unit want to date me from the first day of my last period which would mean a due date of 26th June but I know that’s way out as I have 60-70 fab cycles and I got the negative on 15th November.... so based on conception date it’s more like 28th July! It’s really frustrating me!