Contraceptive injection.......... please help?

Hey I am on the contraceptive injection BUT I have only had it done 2 times because I only started getting it 4 months ago. But I have been told that it can stop you from having babies for 2-5 years, but the nurse never told me this when she give me it, and I am actually felling ready for a baby very soon. Because I havent been on it long do you think it will come out of my system faster? Please give me some advice. Thank you