2nd Floor toilet
Two reasons I can think of. First is a clog in the drainline. You may have to run an auger down the line. Second (because of the glugs) would be a blocked vent. This time of year, squirrels drop nuts into the vents, birds build nests and spiderwebs catch leaves and block the pipe opening. If you can safely go up on the roof with a flashlight, look into the pipe and see if it is plugged. If so, remove what you can by hand and then flush out the pipe with a garden hose.
If you have never made a mistake, you haven't done much.