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1.The history and symptoms suggest the most possible cause could be the
ganglion from the joint space with infective aetiology.
2.Ganglions generally arise from ligaments, tendons, bursar,
nerve sheet. Usually painless, unless its infected.
3.The treatment in this case is monitored when symptomatic, as you said soreness you have to get in
incision and drainage with a course of antibiotics.
4.The other possible cause could be hair follicular infection with the collection of pus with in - which needs a small incision and drainage followed by antibiotic treatment.
5.I would suggest to see your GP, let The GP examined and take needful decision.
6.As a remedy do not disturb by squeezing or pressing the lump as it will complicate. Good luck.
Hope i have answered your query,
any thing to ask do not hesitate to ask.
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