no period?

Can anyone answer why I haven't had my period? My last one ended September 17th. There is no way I'm pregnant, I've taken 3 pregnancy test and they're all negative. I was on the depo shot for almost 2 years but have been off of it for over a year now. I've had irregular periods before but not this bad. Has this ever happened to anyone else? Does anyone have any ideas of what might be going on?