I take the Contracep correct injection, is there a risk of getting pregnant?

I take the contraceptive injection since 27/02/2011, every 3 months I take the injection always on the 27th, right the last one was on 10/27/2011, I would like to know if there is a risk of becoming pregnant, because my period does not come I have known for a few months that this is normal, but today I felt sick when I became pregnant.