In reply to Hannah V:
A multitude of causes.
How's your technique, good and efficient or just learning. Weaker than desirable muscles might be causing extra stress and strain on the ligaments of the knee, as they work hard to hold your position. Fit of shoes, inner soles etc. Good foot position flat etc. not causing stress to travel up.
As you no doubt know knees are fairly critical to skiing, they have to cope with a lot. If you just double pole do they still hurt after? Patellofemoral pain, or anterior knee pain is just as common with runners as skiers.