Period cycle. What could be the reason.

Most probably hormonal imbalance thyroid weakness

It's not normal to have irregular periods  ( early or late), missing periods, spotting between periods, prolonged heavy flow, pains and cramps during cycle, PMS and mood changes before periods, PCOS, Fibroid, Endometriosis,  fertility issues, tiredness, hair fall,  sleep disturbances, weight issues etc. If it is happening to you,  it's a sign of underlying  hormone  imbalance  which needs further evaluation with blood investigations , scan etc.Good news is that all your problems  can be totally  reversed , not by masking the symptoms  with pills but by addressing  the underlying  cause of the problem and healing it from the ROOT cause which is not in your pelvis BTW. Why is it a serious problem and why you should not ignore it? Because, If it is not treated early, it can also lead to serious health problems like diabetes, heart disease, and even cancers. Not only the symptoms of hormone imbalance make your life miserable, there is a scientific proof that they also  increase your chances of developing a big disease later on in life, like full-blown diabetes, autoimmune thyroid disease, and even cancers. The BioCycle study (NIH 2009) showed that untreated, hormonal imbalances increase significantly a woman’s  risk to diseases of inflammation – heart disease, diabetes, cancer, and premature aging.

Hormones imbalance is common cause.May be she is having hypothyroidism or hyperprolactenemia.Get done ultrasonography to know the pathology of uterus.Also get done TSH and serum prolactin level.