Mixed messages is when someone goes back and forth, back and forth about their intent and desires. Its not necessarily from an intentional or cruel place, though. Usually the person is just confused. This doesn't seem like mixed messages to me. She expressed possible interest, then, what it sounds like to me, is she changed her mind. She hasn't gone back to actions of interest. That's not mixed messages. That's having an interest, then something happens that causes her to lose interest (what that may be, I couldn't say. Maybe she's not a good person, or maybe she thought better of flirting at work. Maybe she started seeing someone else. Maybe something happened with the OP in his car or text messages that turned her off, we don't know. also if this is a "girl" and not a woman, they're young and her behavior makes more sense. Perhaps she was confused herself). We don't know what type of person she is. Its best not to assume.

OP, Its still my opinion that getting involved with a colleague is not a good idea. There are exceptions. This does not seem like one of them though.
Its disappointing for sure to have a crush and to at first have it returned and then not.