Ask eve: birth control

Ok so I started the depo shot 2 months for my birth control because lord knows I can't remember to take those damn pills every morning especially at a convenient time for school and summer. Anyways I haven't had my period hasn't come since then. I know a side effect is irregular periods and my periods were irregular before I started anyways but not to where I would skip months at a time. But a few days ago I started noticing that my discharge seemed to be mixing with what looks like REALLY old blood because it was a gross looking brown and I'm just wondering if all of this is normal??