I'm always very conscientious about paying my fair share, but that doesn't mean that *someone* couldn't have made a mistaken impression about me. But all four of them?
Well, sometimes things get innocously spread among groups of friends, especially if some are better friends than others. Like, say:
Person 1: Hey, did you notice if CC chipped in for the birthday girl?
Person 2: I don't know. I wasn't paying attention. I know sometimes people haven't. That's annoying.
Person 3: *internally mistaken* Hrm, CC didn't pay for the birthday girl.
Person 4: Did you see CC sent an email about new birthday girl's birthday?
Person 3: I know! And she didn't even chip in for the last birthday girl's dinner!
Person 4: REally? CRUD MONKEYS!, that's rude.
Person 5: We should just pay our own meal then,since she stiffed the group before.
Just funky stuff like that happens, especially if no one is willing to just come out and ask people.
Oh, the other possibility is that it got around that the birthday girl stiffs sometimes, by the same process, and people decided to not pay for her, not thinking it would end up coming out of your pocket. Like, well, if birthday girl doesn't pay for other people, I'm not paying for her lunch kind of thing.