Hi again, and thanks for your very helpful reply.
Because Cassie has furry companions, her lip could have been bitten or scratched accidentally, if they were roughhousing or just playing. I'm glad to hear she's eating and drinking normally, as that shows that it's not very painful, but it might be slightly painful.
She could have been stung or bitten by an insect or spider in the house and that would cause the swelling, or her teeth may be misaligned and her upper fang is hitting her lip when her mouth is closed and/or when she chews. Her fang may be loose (upper or lower) and causing this, or she could have a gum infection inside her lip in that area.
Another possibility could be the beginning of an eosinophilic granuloma. Here is more information:
It would be a good idea to have her seen in person by her vet to have this assessed and treated, if necessary, but in addition, because she's losing weight, which I not unusual for older cats, she should have a routine senior blood panel, including a thyroid test, to make sure all her levels are where they should be.
For the moment, you can apply cool compresses to the area, to minimize swelling (if she'll allow).
I hope all will be well with Cassie and please let me know how she's doing and what the vet's findings are. Thanks!
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