Perhaps it is time to ween the kitties...how old are they? THIS IS THE ONLY REASON I HAVE TO OFFER...GOOD LUCK!
Could be mastitis, an infection of the breast tissue that can occur when nursing, she may need antibiotics to get rid of it.
This is one of the symptoms of pregnancy and you make sure that this is something else. The skin infection can also cause her nipples and stomach swollen so it is better to take her to vet to give her proper medical attention.