Confused

I was getting negative tests all last week and this week and come Wednesday I started bleeding but only very very light, Thursday was heavier, yesterday was medium kind of flow. It's like my usual period but not as heavy but I got a positive pregnancy test. I'm confused as to why they've all been negative and Thursday is when I first got a positive. I usually have a 30 day cycle my last period was 17th September so was supposed to be due 17th October but didn't start bleeding til Wednesday. I know I ovulated later. Can anyone help?