Dr B can't tell over the Internet if your bird is sick or just trying to keep warm, sorry.
Many birds fluff up when sleeping and resting, especially when it is chilly in the room. It is their way of keeping warm. If this is the first winter with your bird, it could be that your home is cooler now and the bird is just trying to keep warm.
However, sick birds fluff more often, and since you said this is a new behavior this may indicate your bird is sick. Do not delay in taking your bird to an avian vet, because bird's hide illness until they are very sick.