What is ruining my bathroom ceiling?

Yes it sounds like the toilet wax ring needs to be replaced. They run about $15 at your local home improvement store.

I agree with MarleyGMA! First thing to check is the wax ring on the upstairs toilet.

I agree too. The leak from those is not visible on the upstairs floor and you wouldn't be able to feel that moisture on the floor either. Is the ceiling damp or soft when you gently poke on it. Poking too hard will put a hole in it although you will have to repair it after the leak is fixed.

Just a thought, add food coloring to the upstairs toilet to see if it eventually makes it to the ceiling below. Try a different color in your tub drain. If you know how to change out the old wax ring in the toilet above, do that. If the leak continues, you may need to contact a plumber. Best of luck.

Don't forget to check the bathroom fan to be sure it's venting appropriately and not driping condensation. Also, check the back of the toilet and tank for condensation as well as being sure the stool's water line is secure.

Checking for the source of the leak with food coloring is an excellent idea. The water could be from the tub. Fill it up and see what happens. You might need more sealant around the tub drain. If from the toilet, the likely suspect is the wax ring. If color testing does not identify the culprit, you probably have a leaky pipe and need a plumber. Her's hoping for a defective wax ring.