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1.The causes for the lump under the armpit / axillary region with pain and tenderness suggest infective aetiology.
2.Most possible cause could be hair follicular infection with collection of pus within,
abscess.
3.If you are my patient i would have examined and if necessary ask for an
ultrasound which will give me the nature of the lump its source and extensions.
4.The treatment in this case is a small
incision and drainage followed by a course of antibiotics. And a good post procedural care.
5.Yes you need to seek doctors help as i will not go on its own.
6.Do not touch, scratch, press the area - it will complicate and delay in healing. Good luck.
Hope i have answered your query.
Any thing to ask do not hesitate. Thank you.