What Causes Delayed Periods After Taking A Depo Shot?

I've been off the depo shot now for 3 months and only had one shot which was on the 1st July 2013. Since the shot is supposed to have left my system I have had no bleeding but have been experience frequent dark brown clumpy discharge. It ranges from light spotting to slightly darker and slightly heavier spotting. There have been times when the discharge has stopped completely for about a week but then it returns and it's become annoying and troubling that I have yet to have a period because that suggests to me that I'm not ovulating or my hormones are all over the place. Is this normal ? How long will it take for my period to come back ? And after only one shot of depo provera why hasn't it come back and why am I having lumpy dark brown discharge ?

Your help and advice would be much appreciated. Thank you.