I am 5-8 days late on my mestral period. usually never more than 4 days late? what is the reason?

Do a pregnancy: Test if sexually active. If negative any number of things can cause a delay, if period does not come in another week or two see your doctor.

Don't know: 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.