Have to tell you, the truth is probably a secret.  She may not want anyone to know that her mind has wandered to the bi side of life.  maybe it is just a sexual curiosity.  Why dont you do this....next time you have the situation occur, simply say something like, "you are looking at me like I am on the menu"... Do not embarass her by mentioning this in front of witnesses, just say what you think might be on her mind.  or...you could just... tell her that "I would love to....with you!  You know how I am and I can tell your curious what I am like"  or you could just get closer to her when she is in your face, and see if she stays where she is or moves away.  If she stays there or moves closer, "Nail her with a kiss"  tease her into your web and see what comes of your suspicions.  I am interested to find out what happens..HAVE FUN!!