"Interested" is a terribly loaded word. I think she's comfortable around you and that she feels close enough to you at some level that she has few if any regrets about sharing intimacy and pleasure with you.
Also, as much as we try to put labels on things, people's sexuality just won't fit neatly into our nice little named and numbered boxes. Calling any one person lesbian or bisexual or straight does them no justice.
The upshot of these two observations? Just keep on doing what you're doing, being a good friend. If something emerges out of it, SERENDIPITY! If not, you have a good friend with whom you have pleasant memories of a shared night. Win-win.