Is heated up bleach dangerous?

Bleach contains sodium hypochlorite. The fumes being released are almost certainly chlorine, which as you have observed, is quite hazardous. Bleach usually contains strong warnings not to mix with any other cleaning chemical, as some of them will tend to rapidly decompose its active ingredient and release a lot of gaseous chlorine. I expect applying heat has a similar effect.

You are unlikely to have done yourself any serious harm as you're sitting there typing about it, but breathing any more of it is definitely to be avoided.

Bleach fumes are already unpleasant, and heating will inevitable:

• cause any volatile compounds dissolved in a liquid to be more volatile - what's in it will be cooked out rapidly;

  and

• most any chemical reaction (including bleach degrading into gas, or reactions with contaminants) to proceed much faster (chemists describe the general relationship in the arrhenius equation, which is not linear, though not downright exponential) - whatever the bleach turns into, it turns into at a much faster rate.

The same applies to the aggressiveness of a liquid chemical upon contact - most corrosive substances will be much more than doubly corrosive at 60°C vs 30°C.

Never burn a cloth recently soaked with bleach. By recently I mean 2 days. Also place at least a 50 yd. distance from the fire. I must have inhaled the fumes. I was out for about 24 hours, headaches, low energy, nausea, slight fever, loss of appetite.