late period! please help!!

I had sex with my SO a month ago with no protection. This was very soon after my period. I am usually very regular but I was suppose to start last Friday but still haven't started yet. I have taken a couple pregnancy test and they come up negative but I am still worried I am pregnant or something worse. Can someone please give me some insight? Can I be pregnant and still get negative results? Or should I start start thinking of other possible problems? Can my worrying prolong my period?