I'm a seventeen-year-old girl who has been suffering from unexplained pain in my right knee?

I do not exercise (bad I know) and I had been mostly sick in bed for two days prior to, and the day of the pain having started. The pain has been constant for a full week now. My knee will crack every now and then, and it will 'ache' steadily even if it is in as comfortable position as I can manage to get it in. It hurts to bend and to straighten it, and I walk with a slight limp. It doesn't appear swollen or bruised, and it is hard to pinpoint the pain, but it is mostly on or in my bones. As I've said, I've done nothing that might have injured it, and I've never had a previous knee injury before. Any thoughts? Thank you very much for your time!