I definitely got that feeling with my old girlfriend, I would suggest simply talking to her about it. You could also do something especially romantic to spark feelings, it's not difficult to take her out to / make a nice dinner. Best-kept romantic secret: Costco roses. I think it's like $13-14 for two dozen really nice roses. Can't go wrong.

If you're really happy with her, don't let it make things weird. Communication is key, even though the subject's a bit awkward.