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The bump could be due to bleeding under the skin or in a nearby muscle called a haematoma
, especially if it is dark in colour. This may need to be surgically drained for which you will need to consult your Doctor. If there develops pain, restriction of movement of that knee joint he will definitely need Orthopaedician intervention. Apply hot fomentation on that bump for relief, it may subside in a week or so. Give proper healthy diet. Plenty of fluids.