Due date estimator wrong

I just got a positive on a pregnancy test this morning and I am 2 weeks pregnant. I had my last period starting September 17, 2015, but the app says I am 11 weeks pregnant. I used the regular glow app to teach my periods and ovulation and when I marked that I was pregnant today, it went all the way back to when I started using the app and made that my conception day which is wrong. What am I doing wrong?