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At this age, the most common cause of a lump like you describe would be an abscess. An abscess form when a puncture wound occurs and bacteria enters the wound. The outer skin heals and the bacteria multiplies under the skin creating a pocket of pus. It can take a couple of weeks for the abscess to form though some come up quicker. These puncture wounds can occur from an adult that nips at a dog, a cat, or nail or any injury that punctures the skin. You can read about abscesses here:
Sometimes an abscess will rupture and start draining. Your vet will likely drain the abscess and prescribe an antibiotic such as amoxicillin. In the meantime, you can apply warm compresses for 5 minutes or so a few times a day to help promote healing until you can have your vet check the area.
Now there are other causes for lumps but in a pup this age, an abscess is the most common cause. You can read about other lumps here:
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.