My period is 8 days late i have taken

Patient: My period is 8 days late. i have taken a hpt the day after my missed period and one two days later both were negative. but i know how the body can play tricks sometimes. so i am concentrations mainly on the facts. that is my missed period but was just wondering what the possible causes , if not pregnancy, of a missed period. just for a little background. i am 36 years old, and i have always had a regular period , to the day always. the one other time it was late , i was pregnant.

Symptoms: Late period, nausea heartburn, dizzy spells