I've noticed that my BG readings are higher when it's cold (I live in a cold house). Yesterday I discovered that if I warm the meter by putting it in my pocket for a few minutes, the readings are much more to my liking.
Try just warming the strips on their own. The strips contain the enzyme used to sense the glucose, and this may be temperature sensitive. The meter should have been designed to meet normal temperature conditions and compensate itself electonically.