They can get inpregnanted again within a few days (I believe) of having a litter, which is why you should separate the male and female the day she had her pups. Otherwise, look forward to another litter. Mom may kill one of the litters as she will not be able to feed them both.
Yes they can become preg within a few days of having a litter. However they can also put off having that litter until the others are several wks old that way they dont have another one while nursing the first one. These guys do best in a family group not being seperated unless you dont want anymore babies. Also they will not breed more than the area they live in will allow.