Dude, 95% confident she got mad because she's stressed out, not because of anything you specifically did or didn't say.
But in general I find that it keeps grumpiness-prone women a lot happier if, instead of trying to fix whatever problem is "causing" their bad mood, you offer only sympathy and ask them if there is something you can do to help. As opposed to coming up with a solution on your own and offering it to them unsolicited.
For some reason it just seems to be a super-common female/male source of miscommunication, where women are looking for validation or emotional support and men instead try to solve a problem for them.
So if it were me (and I was thinking straight, which I often don't, with often-poor results), I would have written something like "Cool that lunch sounds really good! Anything I can do for you to help out?" and leave it at that.
Just my several cents' worth.
EDIT: LOL like 5 people responded more succinctly in the time it took me to write that. Too many wordz!