You had sex on the 23rd of September, which was 20 days after your first day of your last period? And you are thinking that possibly the sex produced the pregnancy for which you got a positive on a home pregnancy test on the 29th? (Which should have been the first day of your next period if you had had one?) How consistent are your periods? Have you had an ultrasound yet? Did they give you a due date from the ultrasound?