Something under my floor is making my carpet wet, and its spreading! What could be the cause of this?

It started off as a small wet spot in my hallway and I was confused because I didn't remember spilling anything. A couple days later I walked over that spot and my foot got soaked! Now the whole inside of the hall closet is wet, and half of my hallway is covered up by towels! My washing machine is next to the hall closet, but I have inspected all the lines and the bottom of the washer and nothing seems to be leaking. I am in a basement apartment, and it looks like this has happened before because the bottom of the drywall in the closet is deteriorated. I would like to figure out as much as much as possible before I talk to my landlord about it, because she won't know what to do either! We would at least need to know who to call....