It's called a post-partum pregnancy. The female rat goes into heat within 24 hours of giving birth and if the male was still in with her she likely conceived. The eggs delay implantation for a bout a week, so that the gestation is actually 4 weeks or so, to give the current litter a chance to survive. Back to back's are very very hard on ratmom's and the babies. The 2nd litter is usually smaller than the first (size of pups, not amount), since she's nursing her current litter, and trying to nourish the litter in her belly as well. Mom needs a lot of help with this type of situation. BUT you are saying the first litter is only 3 weeks old and now she just gave birth? That's not right. Its more like 4 weeks after the first birth.