Due dates changed

Lee • Hello I have policsystic ovouries, I had a lap and they found cysts on my ovouries . I`m on clomid now 2nd month
At my dating scan at 7 weeks 3 days my due date was 29th October and at my 12 week which I thought I was spot on 12 weeks yesterday turns out I was 12 weeks and 2 days . And my due date is 27th October . It's confusing me so much that it has changed