When can I be due?

So I’m sure you all have been through the same thing. Having PCOS not having normal periods and not ovulating properly. Well here’s my story, I got a positive pregnancy test this past Thursday. And it was very dark and happened instantly. But I took a pregnancy test a week before and it was negative. If I go by my last period it says I’d be a little over six weeks already and if I go by my ovulation which I don’t even know when it happened happened because I never got a positive test as per the app I use I should be at least 5 1/2 weeks. But I didn’t get a positive test until Thursday which is four days ago and because of Thursday I’m just feeling like how could I be five or six weeks? When I was pregnant with my other kids I got a positive pregnancy test at four weeks