Something that might strike you as odd is if you go to take a shower and there are bubbles coming up in your toilet. While this might unnerve you, it is due to a quite common problem known as venting. In a plumbing system, there needs to be a vent for air to escape through, but if this vent is blocked or if there is a blockage in your plumbing system somewhere else, the air will find another way out. Many times this will be through the toilet, therefore causing it to bubble
To resolve this problem, you need to remove the blockage. If the blockage is in the vent pipe, you might want to check if a bird or rodent has built a nest inside the pipe. If the blockage is inside your pipes, you should be able to clear it up using a snake or plunger. Just be careful so the pressure from plunging doesn’t cause water to back up in the toilet.