Although women who are breastfeeding might not have a period, they can still ovulate and get pregnant.
So, if you’re not looking to get pregnant, but you're lucky enough to have the energy and the hots for your partner right now (even though you're probably sleep deprived and craving a little time to yourself), then please be sure to use some form of birth control.
And if you're thinking about some cuddling and some intimacy with your partner, don’t forget that most women who are breastfeeding find that their vaginas are very dry and don't stretch like they used to. So, play it safe and lubricate, lubricate, lubricate. Likewise, their sexual response might be slower. This is completely natural and a side effect of breastfeeding. You see, the hormone prolactin that makes milk inhibits the hormone estrogen, which helps keep our private girl parts moist and able to stretch. So, no matter what you use for birth control, get yourself some lubricant and have fun.
For more on breastfeeding, see my Personal Guide to Breastfeeding.