There are several menstrual problems that can occur in girls under 20, including:

Irregular periods: It is common for young girls to have irregular periods during the first few years after menarche. This is because their hormonal balance is still adjusting and their menstrual cycles may not be consistent.

Painful periods: Many girls experience cramping and discomfort during their periods. This is caused by the contraction of the uterus as it sheds its lining.

Heavy periods: Some girls may experience heavy bleeding during their periods, which can be caused by hormonal imbalances, fibroids, or other underlying conditions.

Missed periods: If a girl misses her period for several months, it could be a sign of an underlying medical condition such as polycystic ovary syndrome (PCOS) or thyroid problems.

Premenstrual syndrome (PMS): Many girls experience symptoms of PMS, such as mood swings, bloating, and breast tenderness, in the days leading up to their periods.

Menstrual disorders: There are several menstrual disorders that can occur in girls under 20, including amenorrhea (absence of periods), dysmenorrhea (painful periods), and menorrhagia (heavy periods).

If a girl under 20 is experiencing any menstrual problems, it is important to seek medical attention. Treatment options may include medication, lifestyle changes, or surgery, depending on the underlying cause of the problem.