Short-term high humidity due to rain

If you have a crawlspace with an earth bottom, it's possible the humidity is coming up through that--check whether you have a vapor barrier below your house. You probably don't, given the age of the house.

Getting a dehumidifier is definitely a possibility. Or a 'swamp cooler' arrangement, where cold water is used to make condensate that you then pipe out of the house, but that really only works in higher temperature air.

(edit) You may find some of this writeup really helpful.

Definitely get the Landlord involved. Unless there's a standing pool of water, much bigger than the toilet, there's no apparent nor logical reason for any room to stay so noticeably different.

I'd say maybe a backdraft through the exhaust fan from a faulty damper, but you've disproven that. In the meantime, check with the neighbors & see if either are doing anything strange or noticed the same thing.

I'd suspect the 1st floor got a Hot Tub or started Indoor Gardening. A Room Humidifier or Air Conditioning System's Humidifier wouldn't even do anything like your saying.