depo injection

I have the depo injection and I've had it for about 2 months now. I've recently started producing like a very dark brown discharge, I have just finished a course of antibiotics for a water infection and it started half way through the course and it's been happening ever since and it's now been a week, I'm worried this is like a sign of pregnancy or miscarriage although I am on some form of contraception. Someone please put my mind at ease