“Fine” has about nine meanings, and your voice tells everyone which one you picked.

Short answers do not conceal anything, they move the message into tone, where anybody who knows you reads it instantly and accurately. Saying fine while sounding flat communicates more clearly than a paragraph would, and it also removes any chance of being answered, since nobody can respond to something you have not admitted. It is a door held shut while you stand visibly behind it.

Decide whether you want to be asked again or left alone, and say which. Not now is a complete answer. If you do want to talk, two more words will get you there faster than the tone will.