If they vomit and their stomach is empty, it's going to be clear and watery. My cat does it from time to time. However, non-hairball vomiting can be a sign of various medical conditions (hyperthyroidism, for one). When is the last time your kitty had bloodwork? One incident of vomiting isn't an emergency, but if she hasn't had bloodwork in the past six months I'd have it done pretty soon. You really need to monitor them closely at that age (my cat is 13 too).