Gary is correct.
"I am fine" is often just a polite response. Maybe I'm sick, maybe I'm not. For instance, I may be as sick as a dog, but if someone asks me "How are you?", sometimes I may just reply with "I'm fine" as I don't wish to talk about how I really feel.
So, a response of "I'm fine" is often just a polite conversational response, rather than an indication of how you really feel.