One month missed period and negative hpt?

Irregular cycle: Many things can cause changes in the regularity of your cycle besides pregnancy. It is not uncommon for cycles to be irregular for the first couple of years after starting. Additionally, illness, stress, excessive exercise, excessive weight loss and diet changes can cause changes in your pattern. If your cycle does not return soon, see your doctor for 'secondary amenorrhea'.

Possible causes: 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.