Period 1 week late, 2 hpt neg. diarrhea. left lower abdomen hurts. used protection. irregular period. why?

Note sure.: Missed/late period: low body wt, obesity, weight ^ or v, ^exercise, endurance athletics, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, perimenopause, just starting menstruation, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e., polycystic ovary syndrome). #1 reason is pregnancy.