Neither, and this is the one species question where the answer is blunt. Guinea pigs do not slow down for winter. They do not hibernate and the gut only works while food is moving through it, so eating less is not a season, it is a symptom.
Cold-kept pigs often eat MORE hay, because chewing and digesting it is how they make heat. So a pig in an unheated shed leaving hay in a cold snap is going the wrong way, and ignoring a favourite vegetable on top of that is the bit I would act on today.