Halim66: Actually, Zack sounds to me more like a timid/nervous cat than a cat with a behavioral problem for the vet. Behavioural problem as per my Blackie's case is not timid or nervous, but a real problem, as in, a sudden change in character/personality. He became totally different, and that was why I knew something was wrong.
The vet will give you more info if you ask them more questions.
han73: I agree with June. Your cat sounds to me more like she doesn't like your other cats. Some cats are aloof, some are friendly, and like June said, some do not like other cats. I personally know one cat which loves humans, any humans, including complete strangers, but can't stand other cats.
It's a behavioral problem for a vet if you see a sudden change in character....cats hide their sickness well, so if you see a sudden change in character, even a simple thing like refusing to eat, then it's time for a visit to the vet. If they have been behaving like that all the while, then that's their character/personality.