pregnancy

Hello everyone I am confused.... My last period was on the 2scnd of June till the 5th. I took my pregnancy test on the 12 of July sincey periods were still not coming and I was pregnant. So i had to start my period again on the 5th of July but none happened. In my mind I started calculating that from that day when I was supposed to start my period as the first week, so according to me am 3 weeks pregnant today, but on this app it says am 7 weeks and 5 days. Plz guys help I don't understand how.