Im stressing out.

GoodMorning, I have a question well I'm unsure when my period was over, but I know that my last period was July 3rd. And I have really irregular periods because sometimes I used to get 27days cycles amor 24days cycles and i really don't know when is my fertile Window is and when I'm ovulate and I'm not going to get pregnant.! Thursday 11th it sayed I was 27% of risk of pregnancy and I didn't know he came inside me and then I took a pregnancy test Friday well last Friday and it was negative and then i took other pregnancy test Sunday or Monday because I took like 3 am.so I'm very worried that I might be pregnant which I really don't want to be right now..