What Causes Brown Vaginal Discharge While On Birth Control Pills?

I have started taking my birth control pills continuously due too being really anemic. I was suppose to start my normal peroid last week, but instead of taking the placebo pills the last week. I needed to start my new pack of birth control pills. So this week would be the second row, in my pack of pills. I started having a little thick brown discharge the last few days. Is this normal and how long can I expect to have this?