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.
I'm sorry I missed your reply earlier.
As long as she isn't running a fever, seems to feel well otherwise, and this isn't growing rapidly you can wait until she starts weaning her youngsters to have her examined. 3 weeks or so isn't going to make much of a difference in most cases.
My hope is that even if this is cancer it is a solitary tumor that has not yet metastasized and removal would be curative.
Thank you for the picture but it is very, very blurry.
What I can see is an irregular, lumpy, mammary mass. It is quite large for a cat, and that is very concerning as size has a lot to do with prognosis in cat mammary cancer patients. Cats that had their tumors addressed when the tumor was smaller were much more likely to do well long term. That may be because small tumors are more likely to be of the benign variety whereas malignant tumors are aggressive and grow quickly. It may also be because if a tumor has time to get big it also has time to spread.
I would encourage you to have her examined sooner rather than waiting for the kittens to be weaned. You may need to wean the kittens early and/or bottle feed them so she can get healthy.
Absolutely. Until cancer affects organ function or the ability to breathe, or allows a secondary infection to take hold, they feel pretty great, and seem their normal selves.
She can still snuggle her kittens and play with them, but she may just not be able to feed them.
We can start weaning at 4 weeks and they can be fully on solids by 5-6 weeks. Not ideal, but maybe what needs to happen.
Let's have her feed them so they are satisfied for a couple hours, then get her examined and see what her veterinarian says.
I cannot say for sure, but unfortunately it does look like a mammary tumor. My hope is that it is the more benign type, and has not invaded deeply into tissues or spread to other sites.