Why is sewage and disgusting water backing up into my shower?
Typically showers are the first to start backing up because they are the lowest point in your plumbing. So the first thing to do is run outside and open the cleanout. The cleanout is a homeowner's best friend.
If nothing comes out, then the blockage is located between your home and the cleanout. Try running a snake down your shower drain to clear the blockage.
But if sewage and water overflow out of the cleanout or standing water is present, then the blockage is located between the cleanout and the city sewer system or your septic tank. Immediately stop using water and call a plumber. That'll save you a lot of trouble and a huge mess on your floors.