I have three kids 15, 13, and 9 all suffer from swollen sore knees and have been diagnosed with Oswald Slatter's disease. It's when the bones grow and put the tendons under pressure because the muscles haven't grown enough yet. I'm explaining it badly. It's very common in growing children, especially sporty kids (mine all play soccer, compete at athletics, dance etc) Has your daughter had a recent growth spurt? It could be the same thing. Treatment is rest, ice, elevation take an anti inflammatory at night for pain. And they grow out of it once they stop growing. Sometimes we get phsio if the pain doesn't settle after a few days. They have to really stretch before exercising as well, especially if it's cold. I hope it's the same thing.