been on birth control for 6yrs and now iam pregant.

I was on birth control for about 6 1/2 years, and when i went in to the school clinic to get my depo shoot the nuse told me wait until i got my records in from my home town, while i was wating my four days i got pregant,, i just wanted to know how this was possible when other women who have been on birth control for three years and have been trying to get pregant for the past year or so..i get pregant so fast.