Pain Behind Knee when straightening Leg from full bend.

Patient: I am a 40 year old male and am mildly active (mild compared to my younger days). I play and coach basketball, but do little more than that.The area “behind” my knee has been hurting now for a week or two. It seemed to start from keeping my feet up on a stool when sitting, and crossing my legs at the ankle. It felt as if I somewhat hyperextended my knee doing this.I can walk, run, jump, side shuffle without pain or issue. Only when I cross my legs at the ankle does it hurt. The worst pain comes when I sit with my legs fully bent and I then go to straighten my leg. That’s when the pain is at it’s worse.Any help woudl be appreciated.