A missed period does not automatically mean that you are pregnant. Missed periods happen for other medical reasons aside from pregnancy. Factors related to your health or your lifestyle may affect your period. So, before you take a pregnancy test or if you already ruled out pregnancy, here are some of the possible reasons why your menstrual cycle is off schedule. 

• Changes on your weight
  Women with low body weight and have eating disorders, like anorexia or bulimia, may experience missed periods. Same with women who are overweight or obese. Significant changes in your weight can change the way your body functions and may cause hormonal changes that may affect your ovulation. Getting treatment and proper diet will help put your period back on track.
• Polycystic Ovarian Syndrome (PCOS)
  PCOS is a hormonal disorder that may cause missed periods. It results in hormonal imbalance in the body that can make ovulation irregular. Doctors usually recommend taking birth control pills for PCOS to regulate menstrual cycles.   
• Stress
  Yes ladies, stress can affect your menstrual cycle. Chronic or prolonged stress can affect your body’s production of hormones. Your emotions can affect the hypothalamus, the region in the brain that controls the pituitary gland. The pituitary gland regulates the hormones that trigger ovulation which is why it may lead to late or a missed period. 
• Use of birth control/contraception
  A side effect of some contraceptives like birth control pills, IUD, Implant or DMPA, is  very light, late, non-existent, or irregular periods. 
• Breastfeeding
  Breastfeeding may disrupt your menstrual cycle, especially during the first few months after giving birth. In fact, breastfeeding can be a temporary contraceptive method called Lactation Amenorrhea Method (LAM). It’s for women who have not gotten their period yet and are exclusively breastfeeding their babies who are less than 6 months old. As the baby starts eating solids and less milk is consumed, your period will eventually go back to normal and you will need to use other contraceptive methods to prevent pregnancy. 
• Perimenopause
  Perimenopause or menopausal transition is the time where a woman’s body naturally transitions to menopause which ends their reproductive years. During this time, irregular periods can occur along with other symptoms like hot flashes, vaginal dryness, lower sex drive, breast tenderness, and fatigue. 
• Illness
  Some illnesses can interfere and affect your menstrual cycle like thyroid disease, pituitary tumors, diabetes, and liver dysfunction to name a few. If you have a missed period and have a chronic illness, it’s probably your illness that’s causing the irregularity. 
• Changes in your sleep schedule
  Did you recently switch to the night shift at work? Shifts in your body clock can actually affect your menstrual cycle. Your circadian rhythm or your sleep cycle can lead to unpredictable periods. It may cause your monthly period to start earlier or later than expected.