What Causes A Delayed Menstrual Period After Having Non Penetrative Sex?

My boyfriend was getting himself off on the seventeenth of this month, and it was right at my entrance. We re both virgins still but he got pre-cum around the area, and now my period is two days late. I took a pregnancy test yesterday and it came back negative and I have a second to take at the end of the week if I don t start by then. And basically I don t know what to do at this point, should I be worried?