My Female Pit has a swollen breast

Question
Hey.. my dog gave birth to 9 puppies last Tuesday.  They are currently 11 days old, and most of the areas around her nipples are fairly small, normal sized.  But one is 4 times bigger, looking very swollen, and I am concerned.  Is this just because the puppies are using her other nipples (she has 10) or could there be a problem?  Please let me know if there is anything I can do, the vet is not open for a few days!

Answer
Hi Jenny. Please forgive me for not getting back to you sooner. I've had a sick human kid here.

It sounds like it could be mastitis. Is the breast red/warm? What color is the milk? Either way, I'd get her in to the vet today to have it checked out before it gets worse, or spreads to other breasts. Again, sorry for the delay, hope all is well.