Many possible causes. Pregnancy, or not ovulating (releasing an egg) makes your period late by weeks or months. Common causes are polycystic ovarian syndrome (pcos), underactive thyroid, excess prolactin hormone, aging of the ovary (diminished ovarian reserve), or low hormone levels (hypogonadism) from heavy exercise, eating disorders, or stress. See ob/gyn for tests; reproductive endocrinology / infertility md if ttc.
Late period. Pregnancy, not ovulation that month, stress, physical stress or illness. At early age cycles are not always on time. Anything between 2-6 weeks could be due to one of those causes. Athletes may also suppress normal cycles with extremes of exercise.