When my dog was pregnant, the vet told us to watch for mastitis (which refers to the hardening/swelling of the nipple(s). Since it is usually caused by infection, your dog will need antibiotics. You should call your vet immediately and have her treated since she may pass the infection to her young pups (whose immune systems may not be strong enough to fight off the infection).
It sounds like a case of mastitis. Call your vet.
My dog ha da litter 5 week ago and now she have one blue nipple someone told me to give her a hot bath and let a puppie milk on that one to help the swelling and it would not be dangerous for the puppie because they are older is it true if not what sould I do