Knee Pain But No Injury?

[Saraebry]

Hello. So for at least a month I've been having bad knee pain in my left knee only. Initially it only really was bad at night, but as time has gone by, it's been during the day as well, and it's getting worse over time, making it harder to walk normally. It hurts when I bend it, feels slightly better when I straighten it but there's still some pain. I never did anything to it, no injuries or anything. I'm a 15 year old female if that helps. I went to the doctor and had an X-ray which showed no breaks or fractures. I'm curious as to what this could be, especially because of the no injury factor. Thanks so much for your help!