Sometimes your period might come earlier than it should, and there are a lot of reasons for that. Your cycle is controlled by the hormones estrogen and progesterone, so an imbalance of them might lead to some irregularities in some natural inner processes in your body: puberty or menopause. 

Besides, there are a number of external factors that can affect your hormonal balance

• Intense exercise
• Weight changes
• Stress 
• Changes in your lifestyle  
• Travel (especially to the north or south)

Starting or rapidly stopping the intake of some meds such as blood thinners, contraceptive pills or emergency contraception also causes irregular periods. 

Unfortunately, some diseases also lead to irregularities too 

• STIs (sexually transmitted infections)
• PCOS (polycystic ovary syndrome)
• Endometriosis
• Diabetes
• Thyroid disease

Log your period data and additional characteristics in the MIA app to be able to track your cycle. Consult your doctor if you have any reasons for concern. 

Sources:

What Causes Your Period to Start Early?, healthline.com

Early Menarche and Menstrual Problems Are Associated with Sleep Disturbance in a Large Sample of Chinese Adolescent Girls, ncbi.nlm.nih.gov