If you think that you may be pregnant, you should have a pregnancy test. However, when the pill is used correctly, it is 99.8% effective in preventing pregnancy.
Brownish discharge occurs from time to time, even with the use of birth control. It is likely the brown discharge you are experiencing is just spotting related to your body adjusting to the hormones in the oral contraceptive pills.
If your brown discharge is still a concern, you may want to make an appointment with a health care provider for an evaluation. This and other changes related to the fluctuation of hormone levels should clear up within 2-3 months.
Hope this answers your question!
REFERENCE(S)
1. The Answer Spot,
Brown Discharge...NO smell but still freaks me out(Online: Orgeon State University, 2005)
Brown Discharge...NO smell but still freaks me out : SHS AnswerSPOT, Oregon State University