Where on earth could it be? please help girls?

ok so i already asked a similar ques to this, but my period is over 2 weeks late now, and a couple of days ago i took a pregnancy test and it came out negative. i am extremely careful and protective during , I also started college and really stressed out so i figure that also causes delays. i usually am regular, but sometimes a week late every month. what do you think is going on? please help with your opinions. i know im not pregnant, i can just feel it. you think it could be a more serious problem?