What Cause Irregular Period After Coming Off Depo Provera Injection?

hi i am a 18year old teenager who has been on the depro vera injection (3 month injection)for five years .i finally got my periods after a year when i came off the injection.after two four months i went onto the two month injection and have since then went back and forth on and off the injection..throughout this i have had my periods some times i would get them sometimes i would miss two months.I recently had sex end of october 2010 and have not had my periods intill now january 2011 i had taken th emorning after pill aswell as three prenancy tests throughout each month.i have no symptoms of pregnancy no nausea no cravings etc. my father is also diabectic and i wonder if i am to if this could be the cause of my missed periods as i love salt and sugar