1 month late for my period with 3 negative pregnancy test. i am not under any stress, gained any weight (etc, . reasons for missed period) what's wrong?

Possible reasons: 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.

Many causes: Could be thyroid problem or hormonal problem, or other causes.