I would be just like you though, not wanting to upset people but my plan woud be to pretend to be all excited and stuff, talk about what your gonna do etc and that you cant wait its going to be brillant... and then the night before call and say how bad you are, or the morning you go, say you cant stop being sick / the runs... that way she will believe you because you were so looking forward to it, so couldnt possibly be faking!
If you do that, ring her but dont go overboard - if you were really throwing up you wouldnt care, you would just ring and say you were really bad and have to put the phone down...
GOOD LUCK - although hopefully you may have told her anyway by now.