I had this happen to me. the problem was in the main sewer line exiting the house (well downstream of the bathroom). It was partially clogged. Since the shower drain is the lowest point in my house plumbing that is where the water backed up when there was more water going into the sewer line than was able to leave. I thought my septic tank was full so I called a professional. Turned out the line exiting the house was clogged with fat/grease.