To be safe, you might want to use inside her, because that seems more acceptable to English speakers as a whole.
In your clause lit a fire inside (of) her, either inside or inside of are used. In the first case inside is a preposition; in the second case inside of is a two-word preposition.
Note that MacMillan Dictionary says inside of is "sometimes used instead of inside, especially in American English."
Nevertheless, Americans also say inside. In either case, inside her and inside of her are used. Also: I lit a fire inside the fireplace versus I lit a fire inside of the fireplace. Either is fine, at least to many speakers of American English, although presumably speakers of British speakers would prefer the version without of.
Compare the well-worn phrase think outside (of) the box.
There are hundreds of google hits for both think outside the box and think outside of the box and the version with of has been used in such reputable journals as Forbes and many other periodicals.
Inside is not a noun in either version of your clause, because you can't place an article before it. Both the following are not grammatical:
*Lit a fire the inside her.
*Lit a fire the inside of her.
See also tchrist's answer to Is the word 'outside' a preposition or a noun in this context? (you can start reading with the sentence that begins Here's an example that uses outside as five different parts of speech, which is followed by the highlighted box showing five uses of outside; the same uses apply to inside.)