Upstairs bath tub, Downstairs shower

The upstairs bath tub is in. The tiles are set and grouted, the floor is finished, cabinets installed, toilet works... WE ARE READY!!! Only issue now is that when the water is run in the bath you can have a shower in the Kitchen below. Is it because the drain is not connected? Did one of the water lines come loose? Is a connection not set? Only way to find out is to rip open the walls and take a look. Ideally it is the drain as we can access that from the Bedroom closet behind. If it is a pipe connection - that means that we have to rip up the tiles and redo the bath tub.

I am not impressed.