So someone kisses or fucks her and then doesn't want anything more. And she says she feels used.

Why "used"?

It doesn't imply that she's heartbroken or anything, else I imagine that's the word she would use. It seems to typically be about stuff where something sexual just happened without much romance or anything at all. So I think it's fair to say she is not in love or anything.

It sounds more like she feels that he got something from her and she got nothing in return. Like a business transaction.

Which makes me wonder.

Did she not enjoy it, thus feeling like the transaction was unequal (he getting more from her than she from him)?

But if she did not enjoy it, why did she do it in the first place?

Did she do it simply out of the assumption that she would be getting something out of it?