What Causes Inter Menstrual Bleeding?

I am a 33 yr. old female, about 5'0". I have had abnormal periods now for about 9 months. I will start my period, have a few days of heavy flow, like a normal period. Then I will continue to bleed for up to 3 weeks or more. The blood flow that I have varies greatly. Sometimes it gushes for a day, and then the next thing you know I can just wear a panty liner for a week or so. I usually get about a 2 to 3 week break from bleeding in between cycles. I have no pain, no cramps, or anything of that kind. Just the normal little twinges that I usually get a day or two before my period.