So to summarize a few good answers:
1. indoor air is drier in winter, so sweat evaporates faster, cooling your skin;
2. there may be drafts and pockets of colder air near the floor;
3. the walls are colder and don’t emit the usual amount of infrared radiation, so you lose heat due to your body emitting more IR than it receives;
4. you may spend more time indoors and hence have less physical activity, so your body generates less heat.
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