What Causes Brown Vaginal Discharge While On Depo Shot?

I am on DEPO and I just started it on the 14th. I waited over a week before having sex and I am now experience brown discharge and sometimes it has some red in it. It s light, not heavy and I ve had it for a few days now. My period was due a week after the shot. I m not sure if I m pregnant, If I m reacting to the shot or if I m getting my period.