Continous menstruation for months. what can cause this?

Many possibilities: The most common causes of continuous cycle or irregular menses would be dysfunctional uterine bleeding and/or anovulatory cycles. If this just started then fibroids (benign tumors in uterus) can be the cause. Overgrowth of uterine lining ( benign or cancerous) can cause irreg bleeding as well. This is less likely at age 31. Ultrasound can be done to evaluate. And labs to look for other causes.