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This is a common place for fatty tumors. Fatty tumors are usually pretty maleable and feel like they move from side to side under your fingers. They are common in an aging pet. If you see one, you will usually see more.
No matter what, you need to see the vet as soon as you can to have a needle aspiration done to examine some contents of the lump. This will rule out cancer. These lumps are only cancerous in a small number of patients but you still need to have it checked.
Let me know if you still have questions.
I received your PM. If the mass is growing quickly through the day, it's probably not a fatty tumor. I would feel more led to think that you are dealing with an allergic reaction to a bee sting or bug bite.
If you suspect this, you can use a dose of benadryl as an antihistamine if you are comfortable monitoring her at home. The dosage that my vet recommends is 1 mg per lb of body weight.
I am glad that I was able to help. Now that you have found the bite, you can apply some amonia to the area to reduce swelling. Then use benadryl as mentioned above to treat the symptoms. You can use it every 8-12 hours as needed.
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