I am 15 day's late for my mensral cycle and i have taken 4 pregnancy tests and they all are negative... we have been trying to conceive?! what's wrong?

Depends: It is not uncommon to have an abnormal period once. Stress , strenuous exercise, medical problem like diabetes or thyroid condition are a few reasons one might have repeated episodes of abnormal menses. See your md, if appropriate do a blood pregnancy test.

TTC: 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.