Say 'I'm fine'. That's what everyone does, regardless of their actual state. 'Fine' is code for 'I realise you haven't time to actually listen, and to be honest I haven't really time to answer, not honestly and completely, anyway, but thank you for taking a moment to ask, and I'm good enough to be going on with'. All they want is verbal confirmation that you're not currently about to keel over or scream, and they can go on with their day. Not too much to ask, for all concerned. It's not even really a lie, just a little social code indicating that no-one needs to panic.
Some people, however, do want the full and complete answer, but outside of an emergency it's probably not a good idea to give it to them. Remember. Your thoughts are not their thoughts, and some of your thoughts are thoughts they really don't need to know about. That is why social lies exist.
Gods, I need to learn how to lie.