pregnancy tests

Ok so with my daughter I took tests once a week every week. I started the week b4 I missed my period and I did not get a positive, not even a faint line, until I was already 6 weeks pregnant. I even went to the doctors and had them do a test 3 days after I missed AF and it was negative. I finally got my positive like I said I was already 6 weeks along. Has anyone else experienced this? My partner and I are ttc and I'm just wondering if it will happen the same way again? My daughter is 3 now fyi