Can Depo-Provera cause a period to stop within a month?

I started on Depo-Provera about a month ago, and I read that after half a year to a year, many women stop getting their periods. I've had sex since I started the shot, and my period is late. I've never been regular, even when I was on the pill, so I'm not sure if my period being late is because of the shot, but I was experiencing the headaches I usually get when I'm PMS'ing. Is there any way to know if the shot is stopping my period? Can it even stop it that quickly?