Am a week late with my periods , have taken 2 pregnancy tests . both negative. want to know what is the reason for the delayed periods?am very anxious

Late period: 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.