I'm confused

Ok so according to this app I started my fertile window yesterday but I don't ovulate till the 20th, Idk why but I had this wierd feeling today to take an ovulation test so I did and it came back positive 😱 I am so happy because we actually had sex yesterday and today I was really nervous thinking I was going to miss this month. We have been TTC since July and nothing yet, last month I started using opks but never got a positive I was kinda scared that maybe I haven't been ovulating since I hadn't got pregnant or a positive opk, but the test I took today just gave me hope and relief I was starting to get a little scared. So why does the app tell me I ovulate till the 20th could it be my mistake from calculating everything wrong?