-Are you doing okay?
-Not bad. Oh, dear.
-What's wrong?
-Sorry?
-Whenever you say that, it means "Goodness, no way, oh, my god. This is bad. Oh, no. What the hell's going on? Damn it." It includes all of them. I clearly said not bad.
-You don't ask that question to someone who's obviously doing fine. When you ask it to someone who seems to be struggling, and they say they're doing well, they're trying to think positively. "Fine" means that they have no energy to talk about it and want you to stop asking. "Not bad" means that something's definitely going on, but they don't want to explain it to you.