Any vaginal bleeding outside the period is called metrorrhagia. Generally, reasons for metrorrhagia can be functional and organic.
Functional metrorrhagia is caused by hormonal disbalance. Sometimes such disbalance can be caused by birth control pills.
Organic reasons for metrorrhagia can be infection, tumors (benign or malignant), endometriosis, miscarriage, injuries, foreign bodies etc.
You could consult your gynecologist about this.