How can I be sure how she really feels about me?

I would say that this means she likes the way your relationship is going [which is a great sign!] but she's not ready to label you two as boyfriend and girlfriend. You need to go on with your relationship with her like nothing ever happened or like she never even said that. Keep flirting and spending one on one time with her. Then, wait for HER to express interest in you. Wait for her to initiate the conversation of being boyfriend and girlfriend. If she never does, stick to being friends with her and look out there for another girl. Make sure you don't go on asking her out, because she might get annoyed by that. She probably wants to keep up her relationship with you, but doesn't want to limited to just you. She might have her eye on another guy, but she wants you as a backup if he doesn't work out. What I'm saying is, don't throw yourself into this relationship. By all means, keep up with the great friendship you two have, but don't try to MAKE it evolve into something more.