There are several possible causes of knee pain in these positions. Early arthritic changes with thinning of the joint surface is the first that comes to mind. This is more likely in the middle-aged to older adult groups.
Patellofemoral pain syndrome (pain involving the kneecap) is probably the most common in younger people. Muscle weakness or imbalance and postural changes cause the kneecap to slide up and down over the knee slightly off center. Compression and irritation of the soft tissues leads to pain.
If you are planning to continue your skiing activities, it might be a good idea to have this checked out. A specific exercise program may help alleviate the pain and prevent