What can be some reason of a missed period?

Menstrual cycles: It is common for young women to skip a period or have irregular cycles. It is also more likely to happen if you are very athletic, gain or lose weight, using birth control pills, or have an eating disorder. Still, if you are sexually active, you could be pregnant. I would track your cycles to see if it happens often and see your doctor if you notice that it becomes more irregular.

Anovulation: If you missed a period and your pregnancy test came back negative, then you most likely had an anovulatory cycle, meaning your ovaries didn't release an egg. This happens from time to time but if consistent, see your doctor because you may have a hormonal imbalance/stress. Consider a birth control consultation to regulate your periods and avoid unplanned pregnancy.

Pregnancy?: Reasons for missed or late periods can include: low body weight, obesity, marked weight gain or loss, over-exercise such as in endurance athletes, breast-feeding, stress, illness, eating disorders, travel, hormone irregularities, travel, drug use, medications (i.e., birth control) or medical problem(polycystic ovary syndrome or asherman’s syndrome. #1 reason for missed period is pregnancy.