Hi , How are you doing,? It takes few years for the hormonal balance of body to mature, so till then her ovulation & periods will be haphazard. It may bleeding all the days or not at all . You need to consultant gynecologist for evaluation , may need Thyroid profile / Serum Prolactin evaluation & also the Coagulation profile. She will need some hematinics to make up the lost blood. Give her lot of water & nutritious diet to overcome the weakness. Hope I have clarified your queries All the best

Consult a Gynaecologist, this can happen with initial few cycles, after getting some test done, doctor will put her on some hormones