Thanks for your question, my name isXXXXX and I hope I can be of some assistance.
Whenever a dog or cat owner finds a new "lump", or nodule involving the skin or the various tissues underneath the skin, I always suggest that they seek the advise of their veterinarian. Some types of growths can be easily diagnosed by taking a small sample of cells through a process called "fine needle aspiration", and examining them under a microscope. This is an inexpensive test to run at most veterinary clinics, and can be completed within a few minutes. It could turn out to be something benign like a fatty tumor (lipoma), dermal inclusion cyst or a benign skin tumor (histiocytoma, fibroma, ect.), or it could be some other type of more aggressive cancer. (mast cell tumor, melanoma, mammary carcinoma, ect.) Just based what it sounds like it looks like, it could be any of the above mentioned differentials, however by what you describe it is likely a benign dermal inclusion cysts, or some kind of abscess, especially with consideration to her young age.
I would have your veterinarian examine the lump and perform a fine needle aspirate. This will give you peace of mind if it is something benign, and if not, your dog will be able to get the appropriate attention in a timely manner. I don't like to suggest that owners just, "keep an eye on it", with skin and subcutaneous nodules, because if it is a more aggressive type of growth, or even a site of infection, leaving it to grow can lead to further spread and increased difficulty in treatment.
I don't wish to send you into a panic, like I said, it could be a benign growth. But I would suggest having your vet perform the simple test I mentioned above as a starting point. Sometimes an inexpensive fine needle aspirate can give you a diagnosis.
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I wish you and your dog the best, XXXXX XXXXX hope this information helped.
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