It depends on if this is lp and what type of trick that Jackson was doing. The movement of "lungeing" (sp?) forward or up might be aggravating the knee joint. While it may not seem likely to us, the knew can be just enough out of the socket and rubbing. If it is lp, you will want to keep that type of movement to a minimum. I can't imagine that it would be lp as you would have notice more symptons before this.
I bet all the running and playing in the snow has done something but I think that it might just be something that is a one time type of occurrence. Let us know what is up. We'll be sending Jackson some good thoughts and keep him in our prayers it's not anything too serious.
Maggie May is sending Jackson a great big Get Well hug and kisses.