Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl has a golfball sized lump near one of her nipples.
While she may have a secondary infection called mastitis due to improper stripping of milk by her kittens and bacteria on her skin or from their mouths getting into the milk ducts this seems unlikely because it isn't painful. Mastitis is horribly uncomfortable/painful.
Because she is not spayed her odds of mammary cancer are higher than a cat spayed before she started heat cycles. An enlarged mass near a nipple with increased blood supply can definitely be signs of mammary cancer. Unfortunately mammary tumors are more likely to be malignant in cats than in dogs, so I highly recommend that she see her veterinarian as soon as possible.
I understand that she is nursing so that getting her away from the kittens will be a challenge, but in another couple weeks they won't need to eat so often so an examination will be easier to schedule. If you notice she is running a fever though and the area becomes hot and painful that means that there is an infection component, and she needs to be examined sooner.
If you'd like to read more about mammary tumors in cats here is a link to a great write up by Ohio State University's veterinary medical center: http://vet.osu.edu/vmc/feline-mammary-tumors
Please let me know if you have any further questions.