What does it mean if you haven't had your period in a month but get 3 negative pregnancy tests?

Amenorrhea: Not having a period is common in young females. I am surprised you do not understand this or have not talked to your girlfriends about this.. Sometimes you just do not ovulate in a cycle. Other times there is too much exercise, or lack of calories as in too strict of diet. Sometimes stress and other vitamins or supplements. If you cannot figure this out in your life you can consult gyn.

No 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. #1 reason is pregnancy.