I had a pain behind the (right) knee just before Christmas, it was right on the bend (you can see I'm no doctor!) I could feel it when I pulled up with that leg. I eased off the cycling for a couple of weeks and re-adjusted my saddle height (I fiddled around with it about two weeks before the start of my pain, so I asssume the cause was incorrect saddle height) and the pain went away.
I also wore one of those surgical knee supports for a while and that helped a bit.
Biy like D*Alex said - Best to see the doctor if it doesn't cleared up - a pain now could become agony later...