Pre-menstrual bleeding

For the last few years I have experienced pre-period discharge--a light dischage, often dark in color, before the beginning of my menstrual period. Two or 3 years ago this pre-period was about 2 or 3 days long. Now it lasts as long as 10 days or more. My entire period is now taking as much as 1/2 of the month. Any thoughts on the cause?