What Causes Delay In Periods?

..last Tuesday me and my boyfriend that I've had for a year and a half had sex, the condom broke...a little precum might have got in me, but were not exactly sure. I have not gotten my period yet, but it's been messed up lately. I usually get it the first of every month, but in may I got it on the 26. I told my aunt about this and she said to take a pregnancy test every 2 weeks till I get my period. I am not having any symptoms of