Dark brown period every month?? Normal?

First off let me say that I am NOT pregnant.. I haven't had in many months so it is not a possibility. I am on the birth control Ortho Tri-Cyclen Lo and have been on it for about 8 months. Every month I get my period, like clockwork on the same day but it is never the usual blood that I was used to before. It is always really dark brown with some red mixed it. I only 'bleed' for about 2 or 3 days and then I just have this brown, gritty-like discharge for a week afterwards.. What is this? Is this normal from being on the pill or what? For the first 2 months I was on the pill my periods seemed normal. This abnormal thing started around the 3rd or 4th month of being on the pill. Any help would be appreciated!! Thank you!