i'd apply gently pressure and rub. not sure if water will help at this point (water can prevent cramping, but as a curative??)
**if you are riding in the cold air, make sure your knees are covered <i think the CW is 50 degrees or below to cover your knees.
if it never happened before so maybe it was an isolated event (lack of nutritional support prior to your ride, not keeping up with fluids because its not hot yet)
as they say, if you wait till you're thirsty to drink....its too late.