Oh, ilove, that's really tough. Molly would undoubtedly just barge in all guns blazing, and tell her exactly what she thought, but then again Molly isn't real and doesn't have to face up to the consequences of that!
It sounds like the one who texted you is definitely jealous of the way you get on well with the rest of your colleagues, and wants to cause a bit of trouble. But it also sounds like you all know that she a bit like that? If she had started saying something about one of it other colleagues, would you believe her, knowing what she's like? I would probably just ignore it, but then I don't like rocking the boat! And you still have to work with her and your other colleagues. If you talked to her, would it stop her, or make her worse? If you don't react to her, is she likely to get bored soon and move on?
At the end of the day, like richtea says, you haven't done anything wrong, and it's such a shame that someone is making you feel like this, you are so much better than that. You spend a long time in work, and it really should be enjoyable. It sounds like people know what this girl is like, and are unlikely to really listen to what she is saying?
Anyway, I'm not sure thats any help, and it is really tough, but we are all here for you, and you are brilliant .... But a problem shared is a problem halved, or something like that!! X