Not sure what kind of industry you are in, but it sounds like she was just looking for someone to ....* at. I used to work customer service and I know the type very well lol. To be honest, i think it was a bit of a mistake telling her you weren't smoking, you don't need to excuse yourself because you weren't doing what she is accusing you of and shes obviously just looking for confrontation so that will just fire her up even more. In that situation I would have smiled at her, and said "I am so very sorry ma'am, is there anything else I can do for you today?" in a very polite way.
Another thing to consider is it probably didn't even matter that anyone was smoking, she was probably just upset about something and needed to find someone to fight with.
I was going to write the same thing. Part of being an adult is learning to direct your emotional response toward the proper object in the proper way at the appropriate time/place. If you work in customer service for more than 20 minutes you learn that many adults are not adults in this sense.
I worked an array of customer service jobs during and after college and there is a certain type of person that is a spoiled in the sense that they feel entitled to be abusive, that is, to unfairly work out their frustrations on others who are not in a position to respond. Very often it's the infantilized housewife who gets away with this at home or the narcissistic manager/executive type that gets away with it at work.
Once I was working the front desk at the main lodge of a ski resort. A woman yelled at me for five minutes because I had ruined her family's vacation--because the weather conditions necessitated that the ski lifts be shut down for 2 hours. It had nothing to do with me obviously, she just felt like yelling at someone. So I just listened to her and nodded until she felt stupid.
The key with these folks is don't feed the flames and don't take it personally. They're children.