I've been on birth control for about 18 months. been seeing dark brown discharge for 2 weeks. now a week late for my period. what's going on? thanks!

Uncertain.: Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e. PCOS). #1 reason is pregnancy.