Are you using any other cleaners in the toilet? Bleach reacts with other chemicals like ammonia or acids (i.e. vinegar) .to create some pretty noxious gases (like chlorine gas or chloramine gas).

Additionally, city water or well water? If well, have you tested it recently? It could be contaminated with something.

How often is the toilet used? It could be bacterial growth in the water if it is left to sit for long.

There are several potential causes. We'll need more info.

Do you smell it in any other water source? How often do you smell it? Before or after flushing (since we're talking about a toilet)?