I understand where you're coming from. This happened to me before. Basically this girl likes you and is trying to look more pristine and good in order to keep you. I would'nt say she's a slut, more in between, and she thinks she found a good guy so she pretends she's a virgin.
And you know why she does all that bs? Because it works. All women know the more guys passed on her, the least interest you have in staying with them long term.