When I take a shower in about 10min the toilet starts to bubble in the bowl what could be the problem

When i take a shower in about 10min the toliet starts to bubble in the bowl what could be the problem

You have a partial blockage on down the line that's setting up a backflow. This backflow is pushing a bubble of air in front of it. That's what you see in the toilet, This will not get better with age, The solution would be to pull the toilet and snake the line from there. Good luck, Tom