Why Have My Periods Stopped After Delaying My Depo Injection From January To March?

Hi I wanna know i was supposed to go to the clinic on January 15 to get an injection for family planning I m using depo so I didn t went to the clinic on January I went there on march now problem is I last saw my periods on January till now I haven t seen them since last year I was using depo but I was seeing my periods but now they are not coming back what could be the cause because I am not pregnant