I have pain behind my knee, can anyone help?

Localised pain on the right knee, behind the knee on the right side. I get it at particular times, sometimes at work as I am out on a golf course doing a lot of walking. Mostly though it happens if i bend my leg at particular angles. For instance - sitting on a chair and placing the outside of my right ankle on my left knee (the way guys often sit), or sitting on the floor with my legs crossed. Any indications of what could be causing the pain? In both these poses i get a stressful kind of pain in that area behind my right knee.