Okay...I get brown discharge quite regularly. It's most likely just old blood. Phen can make your cycle do all kinds of weird things. Even though you may be on the shot or whatever..and don't normally get your period..you can still have bleeding anyway...especially on phen. I am on the pill and have been getting my period every two weeks. And it sometimes starts with brown discharge.
I don't think you have anything to worry about, but if you are, just ask your doc about it.