When first starting the shot, you are definitely going to have spotting. This is normal, has been said with every website, every pamphlet, EVERYTHING. This is something very common and should subside within 6 months or so. It's just your body getting use medicine. If you have spotting to the point where it's completely unbearable, I'd chat with the doc and see what the options are.
@REBELSTAR: It's highly recommended that you switch to a different birth control after being on Depo for 3 years. This is due to the effects that the medicine can and will have on your body. Your weight also has some effect on Depo and can cause spotting. My best advice: speak to your doctor and start thinking about a different birth control.
I have been taking The Depo Provera Shot since 2011 Mainley to stop My Periods I had my Last one June 21'st My Next one is in September when I first started it I was not having no Periods which I was Happy about now I started Bleeding And it won't stop it started doing that a week or to after mine in July then it stopped and started again I am scared I am 36yrs old I started to lose some weight I haven't gained any thank goodness PLEASE HELP