Knee problems, what should I do?

So lately sometimes my knee will just ache or feels like theres an air bubble in it.And it will just feel weak. (If that makes since.) My dad thinks there may be some swelling. I took some pain reliever yesterday and have been icing it off and on. 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.) Also this has been going for more than a few days but has gotten worse. Do you know what this could be? What should I do? (I'm thirteen)