my yorkshire terrier has had seven pups, and one of her teats is extremely large, hard and lumpy, it doesn't seem to cause her any pain but should i be worried about this

Id be a little worried that its Mastitis.....

Mastitis is an infection of the mammary glands related to heavy milk production and incomplete emptying of the glands. The glands become swollen, hot, and painful with dark red or purple soft spots. Other signs include fever, restlessness, and reduced appetite. The mother may not let her babies nurse, and the babies may then cry excessively, become weak, and die if not cared for by their owners.

In animals with severe infection, hospitalization and surgical drainage of the mammary glands may be necessary. The babies will need to be hand fed a milk replacement formula to prevent malnourishment or starvation while the mother is sick

She is possibly starting mastitis. Get a hot damp flannel (facecloth) and hold it against the breast for a few minutes. Then, when the breast is softer, try to strip out most of the milk from it. With luck this will be enough to 'flush it out' and then you can make sure that the pups suckle from it to stop it recurring. Good luck.
:)