There are many possibilities for the cause of her lump including infection, benign tumors, cancerous tumors, cysts, and hernias to name a few. The only way to know the exact cause is to get a sample of the mass and examine it under a microscope. The easiest thing to do first is to do what is called a fine needle aspirate where a needle is placed in the mass and some cells are aspirated and then examined under the mircoscope. Most veterinarians will do this and examine the sample while the client waits. If the mass appears to be cancerous on the aspirate or if the aspirate is non diagnostic then a biopsy needs to be done and examined by a veterinary pathologist. Did the mass appear acutely or has it been growing for a while? If it just suddenly appeared then it might just be an infection that needs to be drained and treated with antibiotics. Did she have any cuts, bites, or trauma in that area recently? If it has been growing slowly then that makes me suspicious it is some type of tumor or a cyst. Since she is such a young dog, a benign tumor would be more likely than a cancerous.
I would recommend that she be evaluated by your veterinarian for diagnostics and appropriate treatment. Please let me know if you have any additional questions.
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