It's been 36 days since last period, when should i start getting concerned?

Would go ahead ;: Take a pregnancy test if sexually active. 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. #1 reason is pregnancy.