What Causes Prolonged Menstrual Bleeding With Brown Spotting?

I am experiencing prolonged intermenstrual bleeding. The volume of bleeding fluctuates: for three or four days it was light brown spotting but for the past two days has been heavier. I have seen a doctor for this issue before but after two ultrasounds they determined everything was ok and would regulate itself. That was two and a half months ago, but the symptoms are persistent. When should I consider the symptoms serious enough to go back in?