Before you call the plumber, you can do some basic stuff and see if it helps.
in the downstairs bathroom, flush the toilet and see if you hear a gurgling sound coming from shower drain. Then have someone stay there and someone flush the toilet up stairs and listen for the same sound.
If you hear a gurgling sound you have a partial clog somewhere. Start by using the good old plumbers helper, the plunger. Use the proper plunger for the toilet and proper straight plunger for the sinks and showers. It may be just a simple hair clog somewhere. Bathrooms are notorious for this.
If no luck, you may have to snake the line.