When you feel warm/cold you say 'Ik heb het warm/koud'. When something radiates heat or to describe the weather you say 'Het is warm'. So 'Ik heb het warm omdat het warm is buiten'.
Describing a person as 'heet' is also like the English way of using 'hot' to mean 'sexy' - the other day I cycled to work in the sun; I walked into the office and said 'Wat ben ik heet!' and my Dutch colleagues fell about laughing.
No this refers to a woman being physically warm. If you want to say that she is a warm person you can also use "warm" but you would say "ze is een warm persoon". Personally I would not just say "ze is warm" then, but als in English I would clarify that it is her personality that is warm if I wanted to say that.