What's wrong with my knee?

So lately sometimes my knee will just ache or feels like theres an air bubble in it. Sometimes if I pop it the air bubble feeling goes away. It doesn't always hurt but sometimes it does. When I walk up and down stairs my knee/knees will hurt. I'm a klutz so I do trip/fall a lot. And then last summer I think I may have twisted my knee or something but I didn't do anything for. (No wrap or anything.) Do you know what this could be? (I'm thirteen)