I really get a kick out of the pregnancy myths floating around, like you can't get pregnant if you pee after sex, two condoms are better than one, and, of course, that you can't get pregnant while breastfeeding. A good friend of mine actually had to learn this one the hard way — she got pregnant while nursing her 10-month old son!
Using breastfeeding as contraception is known as lactational amenorrhea method (LAM), but since it's not 100 percent effective, you shouldn't rely on it unless you don't mind getting pregnant soon after giving birth. According to the World Health Organization, LAM is 98.8 to 99.1 percent effective while breastfeeding for the first six months, but you can conceive without knowing that you're menstrual cycle has returned to normal since you ovulate about two weeks before your period.
If you're not interested in becoming a mother again so soon, your best bet is to talk to your ob/gyn about an effective and safe form of birth control that you can use while breastfeeding.