27 days late and not pregnant. my last period was november 21 , the first day and ended on the 26. why could this happend?

Not ovulating: It is not unusual to skip a cycle here and there. If it keeps happening then you should have your thyroid, prolactin and FSH levels checked.

Uncertain: Reasons for missed /late periods: low body weight, obesity, marked weight gain or loss, over-exercise such as in endurance athletes, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), travel, hormone irregularities, drug use, medications (i.e., birth control) or med problem (polycystic ovary syndrome or Asherman’s syndrome. #1 reason is pregnancy.