"why do I have knee pain when I exercise or walk and then swells up? " I was swimming breastroke the other day and my knee suddenly popped out of nowhere. It started to swell and hurt severely. I am a young teenage athlete that runs and swims and I also ha

Need evaluation. Pain and swelling in the knee is not normal as you know. This is commonly caused by arthritis or a meniscus tear. You need an exam and and xray to start and probably an MRI to evaluate your meniscus and cartilage surfaces. See an orthopaedic surgeon for this.
You. You need to see your doctor for an examination and testing. You will likely need a test called an MRI to check for any tears in the ligaments of your knee. If there are tears found then a surgery may be needed. For now you should rest your knee and seek medical attention. Good luck.