You can be pregnant since is the most common cause of missed periods. If you might be pregnant, treat yourself as if you are pregnant until you know for sure. Use a home pregnancy test as the first step to finding out whether you are pregnant. Pregnancy is the most common cause of a missed period. If you are not pregnant, other causes of missed or irregular periods include excessive weight loss or gain, eating disorders, such as anorexia or bulimia, increased exercise like in endurance athletes, emotional stress, illness, travel, medicines such as hormonal birth control methods, hormone problems, illegal drug use, problems with the pelvic organs, such as imperforate hymen, polycystic ovary syndrome, or Asher man?s syndrome, breast-feeding and premature ovarian failure. Other diseases such as irritable bowel syndrome, tuberculosis, liver disease, and diabetes can cause missed or irregular periods, although this is rare. However, if any of these diseases are present, you will usually have other symptoms besides menstrual irregularities. Unless you are pregnant, chances are your cycle will return to normal next month.