Or similarly toxic chloramine gas, when combined with cleaning agents containing ammonia, such as Mr. Clean.
If you smell "swimming pool" where no pool exists, it might be prudent to leave the area immediately and take a 1000 mg vitamin C tablet, before making any attempts to discover the source of the smell. (Taking vitamin C after the damage is already done won't help.) If the odor is especially strong, or if it causes any irritation to your nose and eyes, evacuate, and do not return. Call your emergency services number and tell them you smelled a strong chlorine odor.
Don't combine bleach cleaners with ammonia cleaners, ever. Don't even store them in the same place. Also avoid mixing bleach with acids, such as vinegar.