What Causes Irregular Period After Depo Shot?

I am 23 yrs old and always had irregular periods I would go 4 to 5 months sometimes without getting a period. But I got my depo shot in February of 2011 right after I had a still born baby and has been spotting on and off. I was supposed to get my next dose in May of 2011 but I DIDNT go and now after 8 days of missed shot I am bleeding like a normal period.... my question is will this b a normal menstrual or will this b sumthin that's gonna make me bleed for months?