It sounds like the brown discharge started a few days before you were due to take your placebo pills. If so, you're most likely experiencing discharge earlier than expected before your period. Menstrual cycles don't run like clockwork - things like stress, missing a pill or taking it too late, or exercise/diet can have an effect them. It's not uncommon for the amount, the consistency/color, the length, and frequency of spotting/bleeding to vary a bit from cycle to cycle.
If you don't have any other symptoms, keep taking your pills as prescribed and hang in there - things should right themselves soon!
Nothing happens in contradiction to nature, only in contradiction to what we know of it. -Special Agent Dana Katherine Scully