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It would have been better if you had specified the exact location of the lump.
My guess is this some kind of localised collection developed as a result of trauma / inflammation. Though bone fractures
are less likely, it still cannot be ruled out.
The condition warrants immediate medical attention. I would suggest you to visit your doctor at the earliest. If the orthopedic appointment is not possible immediately, a visit to the nearest ER is warranted.
Meanwhile, rest and ice the affected limb. You may use over the counter antiinflammatory drugs such as advil
for symptom relief.
Hope this helps.