What Causes Delay In Menstruation Other Than Pregnancy?

I WAS DUE MY DEPO INJECTION ON THE 17TH MAY 2010...BUT I'V NOT HAD IT AS I WAS TOLD I NEED A BREAK I'V HAD UNPROTECTED SEX WITH MY PARTNER......I'V BEEN ON IT FOR NEARLY 15 YEARS I DID COME OFF IT IN JANUARY 2000 AND I GOT PREGNANT STRAIGHT AWAY I HAD MY BABY 17TH NOVEMBER 2000...COULD THIS HAPPEN AGAIN,IV BEEN FEELING SICK BUT I'V NOT BEEN SICK AND THIS HAS BEEN GOING ON FOR A WEEK NOW.....IV TAKEN A PREGNANCY TEST WHICH IS SAYING NEGATIVE BUT THIS HAPPENED WITH MY SON SO IM NOT SURE WHAT TO THINK AS I KNOW THAT EACH PREGNANCY IS DIFFERENT HE IS MY 5TH CHILD THANK YOU FOR YOUR HELP