Do you need to use a pregnancy test the week you should have your period?

Okay basically, I'm freaking out. Me and my boyfriend were fooling around one day. I was already late on my period when we did, but after, I still haven't gotten my period. Like I completely skipped it. So my first question is, what are other possibilities that could be the reason for me to skip a period? (because honestly, it would have to be very rare for me to actually have gotten pregnant because I have actually never had sex, and even though we were just fooling around, I know how easy it is at the same time Nd there's always that chance even though we are always very careful. okay, also, I did try a pregnancy test (clear blue) and I tried it shortly after the week I should have been on my period, yet the test said I had to try it within 4 days of my period or when I should be on it. The test said negative. So my other question is, could the test have been wrong because I used it too late? Or should I try again when I should be on next month? Also, one last question, if I am pregnant, will I skip next month and the remaining 7? Or would it just be the first month? Please take time to address all the questions as I am very nervous and would like to know if I should be worried. Thanks in advance!