Pregnant

I need help guys. Okay so my periods have been weird since I had my last child in March of 2019. Some months I’ll be 2 weeks late and then have a period. My last period was may the 18 2020. When June came around my period never showed, so I did what I normally do, I took a test every 3 days for a week, and then I took two more. They were all negative. Well Monday I was 18 days late and took a test and it was positive! Does this mean that I’m not as far along as I think? (Based on last period) could I have gotten pregnant after my “said ovulation” from this app? I have took test a day late with both of my babies and it be positive! Why did it take 18 days for this one to show positive? I’m so confused.