I think that she's scared of her sexuality and doesn't want to jump into being in any kind of "relationship" with you because that would mean that she isn't straight. I would take things very slowly with her physically. I see a lot of potential for you to get hurt in this situation. If you get along well and communicate well keep doing that. Do your best not to attach a label to what you are to each other. If things feel right and you both want to take things in a physical direction do it. The way you've described it it's the labeling of the relationship that is bothering her more than actually being in some kind of relationship with you. Let her come to you with the "where is this going" conversation when she's ready to (unless you don't want to wait in which case keep this strictly on a friends level to spare yourself some pain).
Everything will be ok in the end. If it's not ok it's not the end.