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Hello Im a 25 year old female, My issue is that if i go up or down the stairs or jump my knee start feeling numb then comes the pain and then I cant walk anymore and i have swelling in the bottom right part of my knee ( after a couple of hours of walking and a lot of stairs on the way. I can walk even 20km a day every day and not feel any pain my weight is totally fine if its relevant. what can it be ? what should I do?

It seem that you might be suffering from tendinitis of the patellar tendon(inflammation of the tendon) I would recommend a patellar strip, physiotherapy and NSAIDS