Money and colleagues is such a delicate issue, most employees will do anything not to have money transactions between them so as not to create a situation where it's uncomfortable, because you have to see each other every day whether you want to or not, and work with each other.
I personally would ask her how much I owe her and give her the money immediately, with a happy face, then never in a lifetime lend a co-worker a dime again, just to avoid making enemies in the office. It's hard enough for me to keep a job without these things already.
If they don't have change, then they shouldn't order lunch. If after you pay her, she ever suggests ordering lunch together again, you can get revenge the way NTs do: give her a very nice excuse but that it's clear it's an excuse, yet she can't call you on it. Something like: "I'd love to, but I have an upset stomach today so I'm sticking to tea and toast. Maybe some other time." This is what hurts the most, and if something will make them re-think their attitude, this is it. Giving them the freedom to discover, in privacy with themselves and without losing face, how much they lost from behaving badly towards you.
_________________
There are two means of refuge from the miseries of life: music and cats - Albert Schweitzer