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Is it easier to feel 'the lump' via the FRONT [anterior] or BACK [posterior] aspect of the knee?
Are you able to FULLY flex and extend the knee?
Based on the info you provided, I see 3 reasonable possibilities for what this is:
All three are serious Orthopedic conditions, requiring Specialist-Doctor's expertise for proper management, so I highly recommend that you consult an Orthopedic Surgeon soon. A detailed knee exam plus imaging study [such as Knee MRI], plus possible biopsy if needed, should clinch the exact Diagnosis, which is always the FIRST step to getting the appropriate treatment.
Hope this helps--let me know if you have a followup,