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1.History with symptoms suggest that the possible cause could be infective
abscess / collection in the subcutaneous plane.
2.The next possible cause could be
hematoma formation after the trauma.
3.In this case it need to be examined, in case of infective collection in the subcutaneous plane - might need a small
incision and drainage with a course of antibiotics followed.
4.In case of hematoma also needs removal followed by treatment.
5.Small and not complicated cases a simple course of antibiotics with followup will be needed.
6.Yes you need to see the doctor. As a remedy do not squeeze,press,scratch the area ( as you will complicate in that way), maintain local hygiene and use soft clothing which touch to that location, clean the area with antiseptic liquid. Good luck.
Hope i have answered your query,
any thing to ask do not hesitate to ask.
http://doctor.healthcaremagic.com/doctors/dr-ganesh/62888