What Causes Delayed Periods?

pregnancy after depo
hello. my name is taye. i am 23 and i think i might be pregnant but i went to the er but they said no. i already have an almost 2 year old son so i kinda know whats the feeling of pregnancy. i use to be nauseated everyday but now its just every now and then. i havent had a period since april 5th, 2012. i was on depo eveer since my son was born but i never missed a cycle i had got off it for abt 3 months and then only had one shot after that. i dont know why now i just stopped having a 2-3 week cycle. could i be pregnant? is it normal for depo to make your period last for that long?