Irregular Period 

first of all you have to know menstruation cycle every 24 -35 days is considered normal .
whats abnormal ?
1: Intermenstrual bleeding or spotting ( we have to check Uterine Lining )
2.Missed Period : most of the time due to hormonal imbalance ( PCOS) or Pregnancy
3: No Period for more than 3 months : We have to check Brain hormones like prolactin and FSH and Thyroid hormone.
4.Post Coital Bleeding : Any bleeding after intercourse means problem in Uterine cervix and we need to check for cervical cancer or severe inflammation