The depo is supposed to work for 3 months and I got pregnant on it I didn't realize because it was messing with my period, I wouldn't have it for months then I would very light. Id say take a test just to make sure.
I didn't have my period for over a year almost two after being on the depo for 6 months. It does happen and yes its good for 3 months. It's more then likely the effects of the shot. If you concerned I would take a test. :)
If I remember correctly the shot is good for 3 months so maybe just this time you're not getting your period because of it? I would just take a test in case.