What Causes Brown Vaginal Discharge Along With Bad Odour While On Contraceptive Pills?

Hi, can I ask a question? I am under contraceptive called injectibles and this is my 1st 3 months..Since I have started this, I got non stop brown to dark brown smelly discharge. Its not that heavy flow, but sometime can fill over half of my panty liners. I am really bothered. Thank you.