Taking Microgynon. Cycle Has Become Lighter. Pregnancy Test Negative. Suggest?

Hello, I am a 20 year old woman who has been taking Microgynon 30 for four years. Only recently, in approximately six cycles, have I noticed my menstrual cycle changing. They have become considerably lighter and now the blood is brown as opposed to the typical red that I was used to. I know the pill can help reduce menstrual flow, but this has taken almost four years to do so. I have recently done a pregnancy test which was negative. I neither smoke nor drink.