"fine" if someone asks how you are doing. Such an automatic social response might be appropriate for friends, relatives or co-workers you are not particularly close with, but not if the person asking is someone close who is trying to be supportive.
The same caution applies the other way. When you are feeling low physically or mentally, many people will try to buck you up by telling you, "Don't worry, everything will be fine." This is a common, socially acceptable statement we are all taught to make to show support. But the true message seems to be, "Don't tell me that you don't feel good; tell me you're okay."
When you really aren't feeling so good, this kind of support obviously contradicts what you know to be true. At such times it is especially nice to have a close friend or relative who can say, "I'm sorry you're feeling down and I'm glad I'm here for you." For that special someone to be there for you, you have to be able to communicate truthfully how you really feel.
