Congratulations on your baby. You are giving your baby the best gift by breast feeding him. Breast feeding is NOT A NATURAL CONTRACEPTIVE. Many a children have been born because of that belief. Breast feeding does make your periods erratic, so many women do not even know that they are pregnant when they are breast feeding.
I would strongly urge you to use contraception. Oestrogen based birth control pills will interfere with your breast milk, but progesterone based pills do not. You may discuss other contraceptive options with your obstetrician.