20 years old female had one depo-provera injection oct/sept 2010 haven't had a period since and its 2012 why?

Amenorrhea: Amenorrhea (no periods) may last up to 1 year after a depo-provera injection. The fact it has lasted more than a year requires a work-up. You may have pcos (poly-cystic ovary disease) which causes chronic amenorrhea. Please see your gyn/primary care for blood tests and possible ultrasound. They may want to give 10 days of a Progesterone challenge.