What Causes Vaginal Bleeding While On Depo Provera Shots?

Hi, this is a gyn issue on oct 14th my iud mirena was removed due to that it dropped dwn to cervis.I got my period on the 15th.on Saturday the 16th I got the depo pavera shot anytime in the past when I had the shot my period wud be gone within 20mins.now I'm still bleeding its like the 4thday.I wud I no if the med is working if I'm still bleeding