Horrible knee twinge for absolutely no reason?

I was just sitting with my laptop on the sofa earlier, when I felt this sudden and almighty squeezing twinge at the top of my knee. It was fairly painful, but only once, so I brushed it off as nothing and returned to my laptop. Then it happened again, and has been happening randomly for the last 20mins or so. To my knowledge I haven't done anything that could have brought this on, any suggestions as to what it could be?? It hurts!!