What Causes Spotting While On Depo Shot?

hi i've been on depo for 5 my first shot was in august and my last shot was in october and since i been on it i havent had no periods ive been having un protected sex the hole time i havent had no beelding until nov 22 i was spotting and having having cramping and mucus plug for a of couple days and ive been having pregnacy signs i was due for a depo shot jan 3rd but i couldnt go they said i cant get another shot until i get my period and i havent had no beelding but now i've have a light brown spotting for 2 days and its not my period could i be pregnant