What Causes Delayed Period And Stomach Cramps?

Hi my name is Angel i am 24yrs.old and im having serious problems with my period going missing i was due for my period Oct.22nd 2010 it never showed it is now Nov.4th and still have no sign of it normally i would have really bad cramps when my period is due this time around i had no cramps at all which isnt normal for me i went to the hospital on Oct.29th becasuse i wasnt feeling well they ran a (urine) pregnancy test and i was told it was negative i am having unprotected sex and i am not currently taking any birth control i am aware thats its not a wise choice the doctor at the hospital stated i could still be preg. it was just too early to tell im am in need of immediate advice please?