As soon as we learned there's magic in that world, there were theories the fever is a result of that, which would make sense seeing how she's a reincarnator (well, not exactly, but close enough). Typically isekaied people aren't born with no chance of getting considerably stronger. All in all it'd not be a new idea. Unused and untrained magic potential causing trouble, even lethal problems, has been used in fiction before. Still, it's a really scummy world, and thus realistic, if the nobles are trying to keep all about it a secret even from rich merchants. It's kind of annoying it's going on for such a long time for Main.
Assuming it is due to magic and magic is very rare in that world, you'd think some people would be on the lookout for it, actively acquiring people who show the symptoms, to put them to extremely important and profitable work. I mean, it would definitely be used in the competition among the nobles and also between countries. The one with the most mages would be a winner. Even if they wanted to control the knowledge, and the privilege, human greed simply can't be controlled. Just look at this series so far in general: It's full of capitalism, inequality, and greed.
But of course it's also possible it has got nothing to do with magic, in which case this talk was just nonsense.
Edit: I almost forgot: Main's mother, take off your clothes and join the bath. Those little brats can provide no fanservice whatsoever.