Getting chilled creeps up on you. First your fingers freeze, then you get tired and lose focus, and at some point simple decisions become hard. The tricky part: if you are cooling down, you often notice too late.
So ask yourself now and then whether you are shivering, whether you're making mistakes you normally wouldn't, and whether you react slowly. If so, a warm break is due, indoors with a hot drink. Pushing on doesn't help here.
If you have serious symptoms, get medical help.