Upstairs bathroom drains downstairs and floods room

Hi, My landlord is just difficult:madhell: so she will not help or hire a plumber. Whenever our upstairs toilet is flushed it floods the downstairs space under the water heater, my bathroom is just on the opposite side of the wall in which they share the same pipe line. I can hear like a rush of water flood that little space and then leak into our remodeled bedroom which was a garage, thankfully we have tile and have to mop up the mess. When the shower upstairs is on, it trickles thru the same pipe and leaks the same. Where can the problem be ? what if there is a crack in the pipe and when fixed all the water will shoot out the down stairs toilets, my toilet was filled with water, very close to flooding over. Any ideas ?