What Causes Delay In Menstruation Other Than Pregnancy?

my question is this... Last month I started what i thought was my period. I bleed for about 24hrs if that. Well this month I still have not started and i am over a week late. I have took 4 pregnancy test and they all have keep coming back neg. Could i be pregnant or is my period just late, if so why would if be late. my periods have always been on time..