It's not anything. Keep the topics of conversation appropriate (work is fine, but gossip, complaining, bitching about other coworkers etc, definitely isn't) and it's no big deal.
Talk projects, talk customers, etc and she'll be fine.
It's also fine to shrug off lunch, sometimes. Do it wrong or too much though, and she'll come off and "better than thou" and that won't help any office situations at all.
The thing I always keep in mind, is that if you're in a typical 9-5 or worse office situation, you're spending as much if not more time with those people than you are with your family or friends. If you make it adversarial, work will just end up sucking more than it already does<g>