Had you taken a week or two off from freestyle in the recent past?
Because I have had similar pain under my knee two times:
Once after getting injured and soccer and not exercising much for over two months, then once I healed up and started working out again, the first two to three weeks I would get some mild - sharp pain under my knee after working out for a few minutes or freestyling
Then, I recently took two weeks off of freestyle when I went on vacation (and a break from running as well) and when I came back and started trying consequetive atws, after a few hours I felt that same pain, but not very severe.
For me in both cases the pain just stopped coming after I got back into exercising for a while, making sure to warm up slow with some light jogging and stretching.
If you have what I had then I don't think its Osgood Slatters