Well first of all, congratulations on your new babies!
Mum could have a blocked milk duct or some kind of mastitis? Your vet will best advise. Make sure if any antibiotics are prescribed that they are safe for lactating mums. I know there are some antibiotics that are unsafe in rats when they are new mums and for the babies aswell as the drug gets into the milk they are drinking, so do ask your vet about that if anything is prescribed.
Best of luck with the new babies and I hope mum is okay. x