It's a blockage of some sort. My kitchen sink used to fill up the bathtub with a fountain of grey sludge. The plumber found a glass coffee percolator lid bulb in the pipe. It had been there for many decades: possibly as early as the 1950's.
I think you had better call a plumber before you are bathing in..............you guessed it! Sounds like a blockage in a pipe further along. Your upstairs neighbor must have deposited something quite large
It's raining really hard and the water table is rising. I've had rain water make its way into my apartment through the water heater drain but never toilet water in my bathtub.
@RadiantRuby my kitchen flooded with muddy water a few days ago. My city put in 3 giant water pumps on the other side of town that sends rain water my way. I'm not the only one affected. The city is now trying to find a way to reroute the water. But in the meantime my landlord said there is nothing he can do. 😥