It is possible for people who have identified as heterosexual to develop attraction to someone of the sex gender.
Some individuals discover that they are bisexual. This is not a new phenomenon. The human being is capable of falling in love as well as seeking a satisfactory relationship with others. Even in the animal kingdom there are instances of animals copulating or having a playful sexual interaction with others of the same sex (female monkeys for instance had been observed to engage in mutual self stimulation)
The reasons may be many. Some times people just fall in love with the person they feel some sort of attraction to that cannot be explained. Those who believe in reincarnation may say it is because of past relationships; others believe that each individual has a soul mate (gender is not an issue)
Putting speculations aside as to the cause(s), try to view the situation objectively. If you feel comfortable talking to her about this, do so. It is best to confront feelings than bottle them up. Perhaps discussing it with her may shed some light on your own feelings.