Well this is a tough situation, but I'm sure that with some polite conversation you can let her know your reasons for wearing the kilt. It sounds like you have always been able to talk with her so this really should be no different, let her know why you are wearing the kilt and let her know how her reaction to it is affecting you.

As for the upcoming party if you going kilted just try to be sure to be as smartly put together as you can be, I have found even the most skeptical will treat me better if I am well turned out. You might want to forgo the kilt for her party, but that is of course your call to make.