Can you tell me what could be the cause of a missed period besides pregnancy?

Hormonal abnormality: There are many causes of of missed periods, most common cause are hormonal irregularity, caused by stress, medication, or even early menopause, etc. Nevertheless, a pregnancy test should be repeated, as it may be negative in early pregnancy due to false-negative result from low hormone level at the time.

Missed period: Reasons for missed / late period include: an anovulatory cycle in which no egg is released, marked v or ^ in weight; eating disorder; over-exercising (endurance athletics), stress, travel, significant illnesses, oral contraceptives ; other meds, illegal drug use, breast feeding, hormone imbalances and medical problems (i.e., pcos).