What Causes Dark Red Vaginal Bleeding While On Depo Provera Shot?

Hi, i am new to the depo provera shot and my

first time getting it was about days shy of a month, ive started to bleed to where i can no longer wear pantyliners and have to wear pads. its not heavy but its not exactly spotting either, anyway...it is a very very dark red color. could this be my period or just irregular bleeding? also id like to know how to distinguish between my periods and irregular bleeding. thank you