Hello. Yesterday I Was Sitting In A Chair Indian Style,

Hello. yesterday I was sitting in a chair indian style, and when I got up and started walking my knee was hurting me very badly. I didn't think anything of it and thought it would go away. But when I woke up this morning it was very stiff and still having some pain and it is swollen. I did not do anything to cause a knee injury and have never had problems with my knees before. What is wrong?