I shudder to think about the rest of her preparations, if she has been this flaky with you! If a bride has fifteen bridesmaids, she'd better be organized. Did you decide to leave her a message today? I agree with the poster above - explain that for you to work with her, she has to do her part. She may not realize the amount of work this is for you (I know, sounds crazy, but some people think that makeup artists just get to 'play' with makeup and get paid for it- that it isn't really 'work').
If she is still hard to deal with after your call today, I would stay away from it.