Pucca I guess you're quite right. I think the meaning of this sentence is: Not only was she upset, but also offended. She was both upset and offended. I'm not positive, but that's how I see it:)
Well, now that I have thought about this some more, I want to change my vote.
"Not to mention" means "and also this."
"Not to say" can also mean that ("And I'm not even telling you about this other thing..."), but the more common meaning is that you are stopping short of that stronger word.
He was irritated, not to say incensed. -- In this case, he was angry, but
Many, but perhaps not all, uses of "not to say" are ironic. Recall that (using Wikipedia's definition) irony is a literary or rhetorical device in which there is a gap or incongruity between what a speaker or a writer says and what is generally understood.
So when we say things like not to say, which is a claim that we don't want to say something, we often do quite de