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Since the lump is considerably large, u need to exercise some caution. If there is fever, increase in size of swelling, bulging of the swelling while crying/coughing you need to take the baby to emergency room urgently. If there is any unusual signs or symptoms it is better to get medical help. The above mentioned ones are the alarm sings.
If those signs are not there you can judiciously wait. But if the swelling persists it is better to get in touch with your Doctor. Meanwhile if you can send a picture of the swelling I might be able help further. You can upload as a followup consult.