Well slightly noble in it's intentions as your lie was, what you are actually doing is demonstrating passive aggressive controlling indications and that's why she's so lairy with you. You are in effect trying to frighten and manipulate her out of doing something she enjoys, which I'm sure she's thought about and weighed up in her own head,so her problem won't just be that you lied, but that you clearly think your judgement of the situation is superior to hers and that you feel you have the right to lie to her, manipulate her and treat her like a child in order to make hr do something that you want her to.
I know you did all that with the best of intentions, but it's the sort of thing my wife would hit me ocver the head with a mallet for, if I tried it, and she's a very laid back person.
Your gf feels completely humilated by you I'd think, that's what your problem is not a lie in itself.