She's obviously a poor girl going through the painful transformation into becoming a vampire. It's normal for them to bite without realizing it in that stage, and it often mortifies them. As the transformation progresses however, she will bite more often and more willingly. Other symptoms will present themselves, like aversion to light and to garlic. Once the transformation is complete her bites will turn you into a vampire, so I would cut off the relationship before then unless being a vampire sounds like your cup of tea.