Veterinary questions? Ask a Vet for Answers ASAP
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Would you be able to send me a picture of the swelling?
When did you first notice this?
Is she bothered by it?
Is it hard or soft?
What size is it?
Ok, there are a number of possible causes for this. As it appears to have come up quite rapidly it may be a reaction to a sting/bite/scratch or mastitis (inflammation/infection of mammary tissue). Both of these would be painful and I would expect her to lick the area or resent you touching it.
Given her age etc, we have to consider the possibility that it may be a mammary tumor.
Is the area reddened or normal colored?
That makes it more likely to be an infection/inflammation. If it's an infection it will quite rapidly enlarge and can form an abscess that may need to be drained.
There are a couple of things you can do.
An appointment with your vet would be prudent, though this may be difficult with the limited services at the moment.
In the meantime, you can gently bathe the area with saline (half tsp of salt in a mug of cooled, boiled water) and then apply cold compresses to help soothe the swelling.
If you have any concerns, you can return to this chat even after accepting/rating the answer. If there's no significant improvement over the weekend or if she becomes dull/lethargic/off food etc then she should see your veterinarian.