Why am I so hot?
It all depends what you are trying to ask, and the distinction is quite important. In one case you'd be asking about your perception of the temperature on your body (estar) and in the other case you are asking why you are so sexy (ser). Get this one wrong, and you're sure to raise some eyebrows!
Well, maybe you are in Russia, or elsewhere in Europe, where I understand there's a terrible heat wave going on...
¿Por qué soy tan guapa?
¿Por qué tengo tanto calor?
"I´m not hot, I'm just drawn that way!"
Oh, wait... it's supposed to be bad! Oops!