I would take one if you period was supposed to start. Birth control's good but not 100% fail proof! If yorn not pegnanct, I would start using additional protection if you do want to get pregnancy....
Good Luck
Could be the start of your period starting. Brown blood is common during your period because it's old blood.
To rule out pregnancy, take an HPT with FMU and that should give you your answer
I agree with the above posters. I got pregnant while faithfully taking birth control...it is NOT fool-proof and if you aren't pregnant now, I would highly recommend using a condom as well to prevent any more scares like this.
But for now I would say buy a cheapy dollar store test and test just to put your mind at ease. If you aren't near a dollar store, Wal-Mart or any pharmacy will have a slightly more expensive option (usually in the neighborhood of $3-$7).