"how are you doing?" is literally asking just that. You answered with what you had been "up to" recently - that doesn't make sense. A better example would be

"how are you doing?" is literally asking just that. You answered with what you had been "up to" recently - that doesn't make sense. A better example would be your friend asking "what you been up to?" and you answered in very detailed manner when your friend might have been expecting a brief answer like: "not much" or "I've been busy".