Depo Vera and Spotting!

Daisy • Just tracking my monthly days. Not TTC anytime soon.
I've been taking Depo since this February. I usually dont get my period during the shot and these past 3 weeks I have been spotting. It goes from heavy one day, light to the next, and to almost non existent for 2-3 days straight then back to a heavy flow. 
It's getting annoying really. lol 
Has anyone ever had this happen? Another thing of course is the cramps...darn they're bad. I never cramp during my period but I have been experiencing cramps from time to time.