Our ceiling has developed a swelling or bubble in the ceiling and when touched-it literally fell apart and created a hole. It is dry to the touch but happens to be right in line with our 2nd floor master bedroom shower where the plumbing probably comes down. The shower has a plastic floor with a 4 " lip but is tiled from the lip on up. It is a walk in shower with a sliding shower door

What should we be looking for and how do we fix this problem?

Does the shower floor flex when stepped into? Some shower drain are removable from the top, most from the bottom. Since you got a hole in your ceiling, peek up there and see if a nut is under the shower base. What kind of drain pipe is there? Take the chrome cover off and take a pic so we can see into the drain.

Thank you. We will check on that.