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Think about it - did your attitude towards her change after that "first time" ? Did you treat her differently at work ?
She's manipulating you. Probably intentionally, but sometimes a young girl who's got a screwed up home-life or lack of good examples does it without realizing they are doing it. If she's screwed up enough not to realize it, that's even worse...
NEVER get into something with someone you work with. I haven't, but have seen some friends do it and it didn't come out well in the end.