I wish I had your dog in front of me so I could see this lump!
It's very unlikely to be a serious problem as serious cancerous lumps are uncommon in a dog so young.
I'm wondering if this could be something called a histiocystoma which is a benign type of tumor that occurs on young dogs. Your vet should be able to diagnose this with a fine needle test. This type of tumor almost always goes away on its own after a couple of months.
It could also be a tiny abscess from a cat bite or scratch.
A more serious concern would be a mast cell tumor but this sounds unlikely.
And finally it could be a type of cyst that has become infected which is really not a big deal.
I hope this information helps! If you have more questions about this concern, just hit reply and I will respond as soon as I can. I am in between appointments this morning, so there may be a delay before I reply. Or, if my answer has helped you, please hit accept.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.