"Sorry, working" is a completely acceptable response. It sets up the idea that you're still interested in communicating, but can't right now.

If you don't have time to make a quick message like that, then don't answer right now. Wait until you're on break or off work and then pick up the conversation. An answer that raises more questions than it answers is just frustrating for all parties involved, you included since you get more questions :P

(Realized while looking at this that it sounds like I'm addressing this to you specifically. I'm not. I'm just sharing my thoughts on how to communicate that one's at work in general, so sorry if it comes across as accusatory)