Knee pain whats causing this?

Probably some loose cartilage. Its really hard to say for sure without doing an MRI. I say MRI because it shows everything. Cartilage can have small pieces separate and do what your describing. It also could be calcium deposits that break away from bones in the area. I would say if the pain doesnt go away pretty quick, returns, or there is swelling, you should go see a Sports Medicine Dr. The reason I suggest that specialist is because they see knees all the time and would know the most about it.