I always wondered about it because Iz at one point was saying I'm cold mummy but she was at 40.2.
AFAIK, that is because of the temperature of her body relative to the air around her. If she's really hot, it makes the air, the room, seem colder by comparison.

We have seen improvements after a sweat, but not always & not only... sometimes it just goes up & up... then goes.

I do think that I managed to shorten the duration of a tummy bug once (mine) by sweating it out under a pile of blankets. I didn't get as bad as everyone else, and it didn't last aslong but then, I'm not confident enough to do that to my kids