Quick question... which Depo are you on? There are two kinds, that I'm aware of: Depo Provera or Depo Lupron.
I had tried the Provera when I was about 16 for about a year. I had to stop because I could not handle the side effects any longer. I did spot for about 3 months. It usually takes a few months for your system to get used to it.
The Lupron stops your period completely and basically puts your body into menopause, which is the one I am currently on. Hopefully, this is NOT the one you are on! It is a last resort, especially when you are young (I am 23).
Sounds to me like your on the Provera and if spotting is your only bothersome side effect, your body should get used to it soon enough. Go see you doctor anyways, just to be sure.
Hope everything works out for you :)
call your doc to be sure. Thats the safe way to go. However the depo is a proges only birth control and u get spotting & or break through bleeding. Google the side effects of the depo and see if it is what u are dealing with. But if u are really worried call the gyno and ask.