The females go into heat hours after giving birth. But the window frame for that heat is very small. But it is possible for the females to get pregnant and nurse a litter.
So those who do harem breeding will see a pregnant female give birth to a new litter as her last litter is weaning.
And if a female passes the heat cycle right after giving birth without mating, there is no way for the female to get pregnant until the litter is weaned.