What Is The Irregular Brown Discharge While On Microgestin Pill?

I'm 16 years old. I am on the birth control pill Microgestin 1/20. Today I noticed a brownish colored discharge. I have spotted before, but I'm not even sure if this discharge today was bleeding. I took my last pill of the month today, so my period should start in about three days. I have been stressed lately with regular highschool drama and personal problems, I know stress can cause spotting, but like I said, I don't even know if this was 'spotting'. Is discharge like this normal? Is it a sign of pregnancy?