If you've taken your pillls consistently and not missed any, the chance of pregnancy is very minimal...but not zero.
The brown discharge may have nothing to do with anything. Sometimes, it just happens.
Take a test if you don't get your period on time, but otherwise, try to relax about it.
I forgot to mention that I've been having bad cramps, some forehead acne, and a backache. Not sure if that changes anything...but thanks! :-)