Normally, a woman pass between 11 and 13 period each year, in case of having fewer or more, it is called missed period. Irregular period, however, is not too serious problems as many women are worried about. You can see it as a common sign at the beginning of menstruation and maybe last for a several year till it become balanced. 
If you are seeing an irregular period for months, please take is easy to understand if you are having one or more problems following:
- lose or gain an excessive weight.
- have digestive disorder, eating confusion such as anorexia or bulimia.
- do increased exercises, especially in marathoners or endurance athletes.
- suffer from stress, depression which can easily cause your period disorder.
- use drugs prescribed as illegal drugs including marijuana, cocaine, acid, LSD, and so on.
- Problems with the pelvic organs, such as imperforate hymen, polycystic ovary syndrome, or Asherman’s syndrome.
- Breast-feeding. Many women do not resume regular periods until they have completed breast-feeding.
Moreover, this problems can be the symptom of having pregnancy. For sure thing, see your doctor and ask for a pregnancy test then having a good plan with midwife consults.