Late period , negative test

So I’m a bit worried, but also confused if I am hundred percent negative. I had sex on August 29, which was about my ovulation day and he came inside me . So this was like 18 days ago . My period is four days late. I’ve taken three test and they are negative. Today I’m feeling crampy and my boobs feel weird, but I literally took a test yesterday and it was negative. And I have never been this late before.

Do you guys think I’m for sure negative since all tests have been negative? Maybe it’s delayed from stress. ? Is there still possibility I could be positive?