as i've heard, it could be possible for a woman to start ovulating 2 weeks after birth. if you're worried, take a pregnancy test, if it comes back negative, start birth control.
It is said that women who have recently given birth are more fertile than other women, but I'm not totally sure about how true that is. Any time you have unprotected sex you risk the chance of pregnancy. If your period is late, you should take a home pregnancy test. And next time use condoms and use another form of birth control, like the pill, IUD, etc.