Period problems are very common in teenage girls.
Ovulation and menstruation can be very irregular at first, and this is completely normal.
If there is a long time between ovulations, then the endometrial lining can build up to become very thick. Then, the next period, when it comes, can be very heavy and prolonged.
Ultrasound, either trans-abdominally or trans-vaginally depending on patient preferences, can often reveal the cause of the irregular periods and discover how thick the endometrial lining has become.