Nothing like a heap of towels to snuggle in or underneath to make a piggy happy!
Guinea pigs can get cold indeed and even die from it; they tolerate a smaller range of temperatures than rabbit as they come from a region of rather stable climate and hardly any frosts (lower reaches of the Andes). You can see their feet going blue and the piggies hunching up and shivering.
Mine love the cozies, there are loads of places you can get them from now. Buzz and Woody also use their hay to keep warm. We have got snuggle safe heat pads too that you heat up in the microwave, not sur eif you can get them in the US.