I'm on the depo provera shot

I've been taking the depo shot since a couple of days after I had my first child I get confused and don't know when my period starts I've been getting the depo shot since Dec. 2015 I just got my last shot may 31st I get then on tint any comments on how long can I go spotting when it acted like my period? I don't know if it's heavy spotting or a period it just starts when ever