Why Am I Having Frequent And Abnormal Menses?

When should I call my doctor? I experienced frequent and abnormal periods for 4 months this year, and then didn't have a period since October. I now have been experiencing heavig bleedig with large clots for the last eight days. In October, my doctor did a biopsy of my uterus showing no cancer cells. I'm concerned about my blood loss this week and am feeling weak. What point should I call my doctor?