Brown goo instead of period?

im sixteen had first period at 11 it was irreagular VERY! got on depo shoot was on it for three years n never had a period been off for a year now and still no period so doc put me on the birth control pills and now im on my last pill of the period week and started having brown goo on my tampon yesterday morning ...what coould this be? pregnant? because i havent had a period in so long? some one please help!